Method for producing ni-based alloy and ni-based alloy

ABSTRACT

The production method of a Ni-based alloy according to the present embodiment includes: a casting step of casting a liquid alloy which is a raw material of the Ni-based alloy to produce a Ni-based alloy starting material; and a segregation reducing step of performing, on the Ni-based alloy starting material produced by the casting step, heat treatment, or the heat treatment and complex treatment including hot working and heat treatment after the hot working, to satisfy Formula (1):
         where, each symbol in Formula (1) is as follows:       

     
       
         
           
             
               
                 
                   
                     V 
                     R 
                     
                       - 
                       0.294 
                     
                   
                   ≤ 
                   
                     1.27 
                     × 
                     
                       10 
                       3 
                     
                      
                     
                         
                     
                      
                     
                       
                         ∑ 
                         
                           n 
                           = 
                           1 
                         
                         N 
                       
                        
                       
                           
                       
                        
                       
                         
                           
                             
                               ( 
                               
                                 1 
                                 - 
                                 
                                   
                                     Rd 
                                     
                                       n 
                                       - 
                                       1 
                                     
                                   
                                   100 
                                 
                               
                               ) 
                             
                             
                               - 
                               1 
                             
                           
                           · 
                           
                             exp 
                              
                             
                               ( 
                               
                                 
                                   
                                     - 
                                     2.89 
                                   
                                   × 
                                   
                                     10 
                                     4 
                                   
                                 
                                 
                                   
                                     T 
                                     n 
                                   
                                   + 
                                   273 
                                 
                               
                               ) 
                             
                           
                           · 
                           
                             t 
                             n 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
         
         
           
             V R : Solidification cooling rate (° C./min) of the liquid alloy, 
             T n : Holding temperature (° C.) in the n-th heat treatment, 
             t n : Holding time (hr) at the holding temperature in the n-th heat treatment, 
             Rd n-1 : Cumulative area reduction ratio (%) of the Ni-based alloy starting material before the n-th heat treatment, and 
             N: Total number of the heat treatment.

TECHNICAL FIELD

The present invention relates to a method for producing a Ni-basedalloy, and a Ni-based alloy.

BACKGROUND ART

Members used in oil refinery facilities and chemical plant facilities,and geothermal power generation facilities, etc. are exposed to ahigh-temperature corrosive environment containing hydrogen sulfide,carbon dioxide, various acid solutions, and the like. Thehigh-temperature corrosive environment may reach 1100° C. at maximum.Therefore, excellent strength at high temperatures as well as excellentcorrosion resistance is required of members to be used in facilities inhigh-temperature corrosive environments.

There is known a Ni-based alloy containing a large amount of Cr and Moas a material which is usable for such facilities. This Ni-based alloyexhibits excellent corrosion resistance due to containing Cr and Mo.

Meanwhile, the Ni-based alloy contains multiple kinds of alloyingelements. Therefore, in the process of casting the melted liquid alloy,the alloying elements may be concentrated between secondary arms ofdendrite which is generated during solidification. In this occasion,segregation occurs in the Ni-based alloy. In particular, Mo which has aneffect of improving corrosion resistance is likely to segregate. Uponsegregation of Mo, the corrosion resistance of the Ni-based alloydeteriorates.

International Application Publication No. WO2010/038680 (PatentLiterature 1) proposes a method for suppressing segregation in Ni-basedalloy. In this literature, a liquid alloy of Ni-based alloy is melted byvacuum melting. Then, the liquid alloy is cast to produce a Ni-basedalloy starting material. Further, as needed, the Ni-based alloy startingmaterial is subjected to secondary melting such as vacuum arc remelting(VAR) or electro-slag remelting (ESR), to achieve further segregationsuppressing effects. Next, the Ni-based alloy starting material issubjected to a homogenizing treatment at 1160 to 1220° C. for 1 to 100hours. Patent Literature 1 states that as a result of this, segregationof Ni-based alloy is suppressed.

CITATION LIST Patent Literature

Patent Literature 1: International Application Publication No.WO2010/038680

Patent Literature 2: Japanese Patent Application Publication No.60-211029

SUMMARY OF INVENTION Technical Problem

In Patent Literature 1, after primary melting by vacuum melting isperformed and further, as needed, secondary melting such as VAR or ESRis performed, homogenizing treatment of long hours is performed. Forthat reason, when the production method of Patent Literature 1 isadopted, production cost may increase. Therefore, in the Ni-based alloy,there may be another method for reducing Mo segregation.

It is an object of the present invention to provide a method forproducing a Ni-based alloy, and a Ni-based alloy, which can reduce Mosegregation.

Solution to Problem

A method for producing a Ni-based alloy according to the presentinvention includes:

a casting step of casting a liquid alloy to produce a Ni-based alloystarting material, which has

a chemical composition consisting of: in mass %,

C: 0.100% or less

Si: 0.50% or less,

Mn: 0.50% or less,

P: 0.015% or less,

S: 0.0150% or less,

Cr: 20.0 to 23.0%,

Mo: 8.0 to 10.0%,

one or more elements selected from the group consisting of Nb and Ta:3.150 to 4.150%,

Ti: 0.05 to 0.40%,

Al: 0.05 to 0.40%,

Fe: 0.05 to 5.00%,

N: 0.100% or less

O: 0.1000% or less,

Co: 0 to 1.00%,

Cu: 0 to 0.50%,

one or more elements selected from the group consisting of Ca, Nd, andB: 0 to 0.5000%, and

the balance being Ni and impurities; and

a segregation reducing step of performing, on the Ni-based alloystarting material produced by the casting step,

heat treatment, or

the heat treatment and, after the heat treatment, complex treatmentincluding hot working and heat treatment after the hot working, tosatisfy Formula (1):

[Expression 1]

$\begin{matrix}{V_{R}^{- 0.294} \leq {1.27 \times 10^{3}\mspace{14mu} {\sum\limits_{n = 1}^{N}\; \sqrt{\left( {1 - \frac{{Rd}_{n - 1}}{100}} \right)^{- 1} \cdot {\exp \left( \frac{{- 2.89} \times 10^{4}}{T_{n} + 273} \right)} \cdot t_{n}}}}} & (1)\end{matrix}$

where, each symbol in Formula (1) is as follows:

V_(R): Solidification cooling rate (° C./min) of the liquid alloy in thecasting step,

T_(n): Holding temperature (° C.) in the n-th heat treatment,

t_(n): Holding time (hr) at the holding temperature in the n-th heattreatment,

Rd_(n-1): Cumulative area reduction ratio (%) of the Ni-based alloystarting material before the n-th heat treatment, and

N: Total number of the heat treatment.

A Ni-based alloy according to the present invention has

a chemical composition consisting of: in mass %,

C: 0.100% or less

Si: 0.50% or less,

Mn: 0.50% or less,

P: 0.015% or less,

S: 0.0150% or less,

Cr: 20.0 to 23.0%,

Mo: 8.0 to 10.0%,

one or more elements selected from the group consisting of Nb and Ta:3.150 to 4.150%,

Ti: 0.05 to 0.40%,

Al: 0.05 to 0.40%,

Fe: 0.05 to 5.00%,

N: 0.100% or less

O: 0.1000% or less,

Co: 1.0% or less,

Cu: 0.50% or less,

one or more elements selected from the group consisting of Ca, Nd, andB: 0 to 0.5000%, and

the balance being Ni and impurities, wherein

in a section perpendicular to a longitudinal direction of the Ni-basedalloy, an average concentration of Mo is 8.0% or more in mass %; amaximum value of the Mo concentration is 11.0% or less in mass %; andfurther an area fraction of a region, in which the Mo concentration isless than 8.0% in mass %, is less than 2.0%.

Advantageous Effects of Invention

The method for producing Ni-based alloy according to the presentinvention can reduce Mo segregation of the Ni-based alloy. The Ni-basedalloy according to the present invention, in which Mo segregation issuppressed, exhibits excellent corrosion resistance.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a Ni-based alloy during solidificationin a casting step.

FIG. 2 is a diagram to show relationship between dendrite in FIG. 1 andMo concentration of Ni-based alloy.

FIG. 3 is a diagram to show relationship between dendrite secondary armspacing DR and solidification cooling rate V_(R) in the Ni-based alloystarting material (cast material) having a chemical composition of thepresent invention.

FIG. 4 is a diagram to show relationship between F1 (=the right handside of Formula (1)−the left hand side of Formula (1)) and the corrosionrate in the Ni-based alloy having a chemical composition of the presentinvention.

FIG. 5A is a microstructure observation image of a Ni-based alloy whenhot working is performed one time at an area reduction ratio of 44.6% ina segregation reducing process.

FIG. 5B is a microstructure observation image of a Ni-based alloy whenhot working is conducted one time at an area reduction ratio of 31.3% ina segregation reducing step.

FIG. 6 is an EPMA image in a Ni-based alloy according to a secondembodiment.

FIG. 7 is a diagram to show relationship between F2=(Ca+Nd+B)/S in aNi-based alloy and reduction area after fraction (%) when a tensile testis conducted at a strain rate of 10/sec at a temperature of 900° C. inthe atmosphere.

DESCRIPTION OF EMBODIMENTS

The present inventors have considered that in order to achieve excellentcorrosion resistance in a high-temperature corrosive environment, aNi-based alloy having a high Mo content is suitable, and specifically aNi-based alloy having a chemical composition consisting of: in mass %,C: 0.100% or less, Si: 0.50% or less, Mn: 0.50% or less, P: 0.015% orless, S: 0.0150% or less, Cr: 20.0 to 23.0%, Mo: 8.0 to 10.0%, one ormore elements selected from the group consisting of Nb and Ta: 3.150 to4.150%, Ti: 0.05 to 0.40%, Al: 0.05 to 0.40%, Fe: 0.05 to 5.00, N:0.100% or less, O: 0.1000% or less, Co: 0 to 1.00%, Cu: 0 to 0.50%, oneor more elements selected from the group consisting of Ca, Nd, and B: 0to 0.5000%, and the balance being Ni and impurities is suitable. Then,the present inventors conducted investigation and study on the method ofreducing Mo segregation in a high-Mo Ni-based alloy having theabove-described chemical composition. As a result, the presentinventions have obtained the following findings.

[Relationship Between Dendrite Secondary Arm Spacing and SolidificationCooling Rate in the Casting Process]

The concentration distribution of Mo in the Ni-based alloy having theabove-described chemical composition has a correlation with the dendritesecondary arm spacing which is formed in a final solidification stage inthe casting step.

FIG. 1 is a schematic diagram of a Ni-based alloy while solidifying in acasting step. Referring to FIG. 1, a liquid alloy in a mold 13 is cooledso that solidification progresses in the casting step. Specifically, aportion in the vicinity of the mold 13 solidifies, and thereby formationof a solid phase 11 progresses. Further, in a liquid phase 10, dendrite12 is being formed in the portion in which solidification progresses.

FIG. 2 is a diagram to show relationship between dendrite 12 in FIG. 1and the Mo concentration in a Ni-based alloy. Referring to FIG. 2, inthe Mo concentration distribution in the Ni-based alloy startingmaterial (cast material) after casting, a portion in which the Moconcentration is high is defined as a positive segregation part of Mosegregation, and a portion in which the Mo concentration is low isdefined as a negative segregation part of Mo segregation. Then, spacingbetween adjacent Mo segregations (spacing between the positivesegregation parts, or spacing between negative segregation parts) isdefined as a Mo inter-segregation distance Ds. As shown in FIG. 2, theMo inter-segregation distance Ds corresponds to the dendrite secondaryarm spacing D_(II). In FIG. 2, as an example, the Mo inter-segregationdistance Ds coincides with the dendrite secondary arm spacing D_(II).

FIG. 3 is a diagram to show relationship between the dendrite secondaryarm spacing D_(II) and solidification cooling rate V_(R) in a Ni-basedalloy starting material (cast material) having the above-describedchemical composition. FIG. 3 was obtained by the following method. Aliquid alloy of Ni-based alloy was melted. Then, the liquid alloy wascooled to the normal temperature (25° C.) at various solidificationcooling rates V_(R) to produce a plurality of Ni-based alloy startingmaterials (ingots) having the above-described chemical composition. Inthis experiment, the solidification cooling rate V_(R) was defined as anaverage cooling rate (° C./min) in a temperature range of the liquidsolution from the temperature at the start of casting to the temperatureat the completion of solidification (the temperature at the completionof solidification is 1290° C.). The temperature of the Ni-based alloyduring cooling was measured by using a consumable thermocouple.

Here, in the present description, a section perpendicular to thelongitudinal direction of the Ni-based alloy starting material isdefined as a “cross section”, and the width of the Ni-base alloystarting material in the cross section is defined as W. When the crosssection is of a rectangular shape, the long side of the cross section isdefined as the width W. When the cross section is of a circular shape,the diameter is defined as the width W. Moreover, in the cross section,a region at a W/4 depth in the width W direction from a surfaceperpendicular to the width W direction is defined as a “W/4 depthposition”.

The produced Ni-based alloy starting material was cut in a directionperpendicular to the longitudinal direction. Then, the dendritesecondary arm spacing D_(II) (μm) was measured at a W/4 depth positionof the cross section. Specifically, a sample was collected from the W/4depth position. Of the surface of the sample, mirror polishing wasperformed on a surface in parallel with the above-described crosssection, and thereafter etching by aqua regia was performed thereon. Theetched surface was observed by an optical microscope of a magnificationof 400 times to generate a photographic image of an observation field ofview of 200 μm×200 μm. Using the obtained photographic image, thedendrite secondary arm spacing (μm) was measured at arbitrary 20locations within the observation field of view. An average of themeasured dendrite secondary arm spacing was defined as a dendritesecondary arm spacing D_(II) (μm). FIG. 3 was created by using theobtained solidification cooling rate V_(R) and the dendrite secondaryarm spacing D_(II).

Referring to FIG. 3, in the Ni-based alloy starting material of theabove-described chemical composition, the dendrite secondary arm spacingD_(II) becomes narrower as the solidification cooling rate V_(R)increases. Based on the result of FIG. 3, in the Ni-based alloy startingmaterial of the above-described chemical composition, the dendritesecondary arm spacing D_(II) (μm) can be defined by the followingFormula (A) by using the solidification cooling rate V_(R) (° C./min).

D_(II)=182V_(R) ^(−0.294)  (A)

[Diffusion Distance of Mo in Heat Treatment]

Suppose a case in which the Ni-based alloy starting material produced bya casting step is subjected to heat treatment. At this time, the Modiffusion distance in the Ni-based alloy starting material can bedefined as follows.

Diffusion equation is defined by the following Formula (B):

σ²=2D×t  (B)

where, σ in Formula (B) is an average distance over which Mo moves intime t (hr) in the Ni-based alloy starting material of theabove-described chemical composition (hereinafter, referred to as adiffusion distance: the unit is μm). Moreover, D in Formula (B) is adiffusion coefficient of Mo, and is defined by the Arrhenius equation ofFormula (C):

D=D₀exp(−Q/R(T+273))  (C)

where, Q in Formula (C) is activation energy of Mo diffusion. Moreover,R is the gas constant, and T is temperature (° C.). D₀ is a constant(pre-exponential factor) of Mo in the Ni-based alloy.

D₀ was determined by the following experiment. A Ni-based alloy startingmaterial having the above-described chemical composition was subjectedto heat treatment at 1248° C. for 48 hours. Then, the diffusion distanceσ of Mo in the Ni-based alloy after heat treatment was determined. Morespecifically, the following experiment was performed. According to themethod, the dendrite secondary arm spacing D_(II) of the Ni-based alloystarting material before heat treatment was measured. After themeasurement, the Ni-based alloy starting material was retained at aholding temperature of 1248° C. At this moment, heat treatment wasperformed for various holding times. After heat treatment, the Moconcentration difference between the positive segregation part of Mo andthe negative segregation part of Mo was measured at a W/4 depth positionof the Ni-based alloy starting material. The concentration difference ofMo between the positive segregation part and the negative segregationpart for each holding time in the heat treatment. Then, the holding timet at which the concentration difference becomes 1.0 mass % or less wasdetermined. Note that all of the dendrite secondary arm spacings D_(II)of Ni-based alloy of the Ni-based alloy starting material used in thetest were 120.6 μm. Since the diffusion distance of Mo is given asσ=D_(II)/2, the Mo diffusion distance σ was 60.3 μm. As a result of theabove-described test, when heat treatment at a holding temperature of1248° C. and for a holding time t of 48 hours was performed, theconcentration difference between the positive segregation part and thenegative segregation part of Mo became 1.0 mass % or less.

Based on the item obtained by the above-described experiment (theexperimental result indicating that when the diffusion distance σ is60.3 μm, if the temperature T=1248° C. and the holding time t=48 hours,the concentration difference between the positive segregation part andthe negative segregation part of Mo is 1.0 mass % or less), Moactivation energy Q=240 kJ/mol in a range of 1050 to 1360° C., andFormula (B) and Formula (C), the diffusion distance σ of Mo at holdingtemperature T (° C.) and for the holding time t (hr) will be as shown bythe following Formula (D). Note that regarding the activation energy,the activation energy value of Mo in the above-described temperaturerange in an austenite steel is substituted for the activation energyvalue of Mo in the Ni-based alloy.

[Expression  2] $\begin{matrix}{\sigma = {1.16 \times 10^{5}\sqrt{{\exp \left( \frac{{- 2.89} \times 10^{4}}{T + 273} \right)} \cdot t}}} & (D)\end{matrix}$

[Relationship Between Dendrite Secondary Arm Spacing D_(II) andDiffusion Distance σ of Mo]

Referring to Formulae (A) and (D), if the diffusion distance σ of Mo inheat treatment, which is defined by Formula (D) becomes not less than ½of the dendrite secondary arm spacing D_(II), which is defined byFormula (A), (that is, Mo inter-segregation distance Ds), it isconceivable that Mo segregation can be improved by heat treatment. Thatis, if the holding temperature T (° C.), the holding time t (hr), andthe solidification cooling rate V_(R) (° C./min) satisfy Formula (0), Mosegregation will be sufficiently reduced in the heat treatment.

[Expression  3] $\begin{matrix}{V_{R}^{- 0.294} \leq {1.27 \times 10^{3}\mspace{14mu} {\sum\limits_{n = 1}^{N}\; \sqrt{{\exp \left( \frac{{- 2.89} \times 10^{4}}{T_{n} + 273} \right)} \cdot t}}}} & (0)\end{matrix}$

[Further Improvement of Mo Segregation by Hot Working]

Performing hot working on a Ni-based alloy starting material before heattreatment will allow the Mo inter-segregation distance Ds to be furtherdecreased before the heat treatment. Because, the dendrite arm grows byextending in a normal direction of the surface of the Ni-based alloystarting material, as shown in FIG. 1. In the hot working, rollingreduction is applied in a normal direction of the surface of theNi-based alloy starting material. For that reason, when hot working isperformed, the dendrite secondary arm spacing D_(II) (that is, the Mointer-segregation distance Ds) decreases compared with a case in whichhot working is not performed. Therefore, when heat treatment isperformed at the same holding temperature T (° C.) and for the sameholding time t (hr), it becomes easier to reduce segregation of Mo in acase in which hot working is performed before heat treatment, than in acase in which hot working is not performed before heat treatment.

Here, suppose that hot working is performed at a reduction of area Rd onthe Ni-based alloy starting material after casting step, and heattreatment is performed on the Ni-based alloy starting material after hotworking. In this case, it is inferred that the Mo inter-segregationdistance Ds decreases by an amount corresponding to the reduction ofarea Rd. Conversely, it can be regarded as that the Mo diffusiondistance σ in the heat treatment extends by an amount corresponding tothe reduction of area Rd.

Taking the above-described items into consideration, when hot working isperformed at a reduction of area Rd before heat treatment, the followingFormula (E) holds based on Formula (D).

[Expression  4] $\begin{matrix}{\sigma = {1.16 \times 10^{5}\sqrt{\left( {1 - \frac{Rd}{100}} \right)^{- 1} \cdot {\exp \left( \frac{{- 2.89} \times 10^{4}}{T + 273} \right)} \cdot t}}} & (E)\end{matrix}$

Based on the above-described study, performing hot working before heattreatment will further facilitate reduction of Mo segregation. Here, aseries of treatments in which hot working is performed, and further,heat treatment is performed after the hot working (that is, a combinedtreatment of hot working at one time, and heat treatment at one timewhich is performed after the hot working) is defined as “complextreatment”. When the complex treatment is performed one or more timesrepeatedly on the Ni-based alloy starting material, Formula (1) holdsbased on Formula (E):

     [Expression  5] $\begin{matrix}{V_{R}^{- 0.294} \leq {1.27 \times 10^{3}\mspace{14mu} {\sum\limits_{n = 1}^{N}\; \sqrt{\left( {1 - \frac{{Rd}_{n - 1}}{100}} \right)^{- 1} \cdot {\exp \left( \frac{{- 2.89} \times 10^{4}}{T_{n} + 273} \right)} \cdot t_{n}}}}} & (1)\end{matrix}$

where, each symbol in Formula (1) indicates the followings.

V_(R): Solidification cooling rate (° C./min) in the casting step

T_(n): Holding temperature (° C.) in the n-th heat treatment

t_(n): Holding time (hr) at the holding temperature in the n-th heattreatment

Rd_(n-1): Cumulative area reduction ratio (%) of the Ni-based alloystarting material before the n-th heat treatment

N: Total number of heat treatment

Here, n is a natural number of 1 to N, and N is a natural number.

The cumulative area reduction ratio Rd_(n-1) is defined by the followingFormula (F):

Rd_(n-1)=(1−(S_(n-1)/S₀))×100  (F)

where, S_(n-1) indicates an area (mm²) of a section perpendicular to thelongitudinal direction (a cross section) of the Ni-based alloy startingmartial before the n-th heat treatment. So is an area (mm²) of a sectionperpendicular to the longitudinal direction (a cross section) of theNi-based alloy starting material after the casting step and before thefirst hot working (that is, after the casting step, and before thesegregation reduction step). When the Ni-based alloy starting materialto be the object of S₀ is an ingot, and the section perpendicular to thelongitudinal direction is not constant in the longitudinal direction astypified by a truncated square pyramid shape, the area S₀ is defined asfollows:

S₀=V₀/L

where, V₀ is a volume (mm³) of the Ni-based alloy starting material, andL is a length (mm) in the longitudinal direction of the Ni-based alloystarting material.

Note that when hot working is not performed, the cumulative areareduction ratio Rd_(n-1)=0 (an as-cast material).

The production method of a Ni-based alloy of the present embodiment,which has been completed based on the above-described findings, and theNi-based alloy to be produced by the production method of the presentembodiment has the following configurations.

A method for producing a Ni-based alloy according to the configurationof [1] includes:

a casting step of casting a liquid alloy to produce a Ni-based alloystarting material, which has

a chemical composition consisting of: in mass %,

C: 0.100% or less,

Si: 0.50% or less,

Mn: 0.50% or less,

P: 0.015% or less,

S: 0.0150% or less,

Cr: 20.0 to 23.0%,

Mo: 8.0 to 10.0%,

one or more elements selected from the group consisting of Nb and Ta:3.150 to 4.150%,

Ti: 0.05 to 0.40%,

Al: 0.05 to 0.40%,

Fe: 0.05 to 5.00%,

N: 0.100% or less,

O: 0.1000% or less,

Co: 0 to 1.00%,

Cu: 0 to 0.50%,

one or more elements selected from the group consisting of Ca, Nd, andB: 0 to 0.5000%, and

the balance being Ni and impurities, and

a segregation reducing step of performing, on the Ni-based alloystarting material produced by the casting step,

heat treatment, or

the heat treatment and, after the heat treatment, complex treatmentincluding hot working and heat treatment after the hot working, tosatisfy Formula (1):

     [Expression  6] $\begin{matrix}{V_{R}^{- 0.294} \leq {1.27 \times 10^{3}\mspace{14mu} {\sum\limits_{n = 1}^{N}\; \sqrt{\left( {1 - \frac{{Rd}_{n - 1}}{100}} \right)^{- 1} \cdot {\exp \left( \frac{{- 2.89} \times 10^{4}}{T_{n} + 273} \right)} \cdot t_{n}}}}} & (1)\end{matrix}$

where, each symbol in Formula (1) is as follows:

V_(R): Solidification cooling rate (° C./min) of the liquid alloy in thecasting step,

T_(n): Holding temperature (° C.) in the n-th heat treatment,

t_(n): Holding time (hr) at the holding temperature in the n-th heattreatment,

Rd_(n-1): Cumulative area reduction ratio (%) of the Ni-based alloystarting material before the n-th heat treatment, and

N: Total number of the heat treatment.

A method for producing a Ni-based alloy according to the configurationof [2] is the method for producing a Ni-based alloy according to [1],wherein

the holding temperature is 1000 to 1300° C.

A method for producing a Ni-based alloy according to the configurationof [3] is the method for producing a Ni-based alloy according to [2],wherein

in the segregation reducing step,

the complex treatment is performed one or more times, and hot working isperformed at least one time at an area reduction ratio of 35.0% or moreon the Ni-based alloy starting material which has been heated to 1000 to1300° C.

In this case, the grain size number conforming to ASTM E112 of theproduced Ni-based alloy will be 0.0 or more.

A method for producing a Ni-based alloy according to the configurationof [4] is the method for producing a Ni-based alloy according to [2] or[3], wherein

in the segregation reducing step,

heat treatment in which the holding temperature is 1000 to 1300° C. andthe holding time is 1.0 hour or more is performed at least one time.

In this case, a total number of Nb carbonitride whose maximum length is1 to 100 μm will be 4.0×10⁻²/μm² or less. As a result, hot workabilitywill further improved.

A method for producing a Ni-based alloy according to the configurationof [5] is the method for producing a Ni-based alloy according to any oneof [1] to [4], wherein

the chemical composition of the Ni-base alloy starting material contains

one or more elements selected from the group consisting of Ca, Nd, and Bby a content that satisfies Formula (2):

(Ca+Nd+B)/S≥2.0  (2)

where, each symbol of element in Formula (2) is substituted by a contentin atomic % (at) of the corresponding element.

In this case, the hot workability of the produced Ni-base alloy isfurther improved.

A Ni-based alloy according to configuration of [6] has

a chemical composition consisting of: in mass %,

C: 0.100% or less,

Si: 0.50% or less,

Mn: 0.50% or less,

P: 0.015% or less,

S: 0.0150% or less,

Cr: 20.0 to 23.0%,

Mo: 8.0 to 10.0%,

one or more elements selected from the group consisting of Nb and Ta:3.150 to 4.150%,

Ti: 0.05 to 0.40%,

Al: 0.05 to 0.40%,

Fe: 0.05 to 5.00%,

N: 0.100% or less,

O: 0.1000% or less,

Co: 0 to 1.0%,

Cu: 0 to 0.50%,

one or more elements selected from the group consisting of Ca, Nd, andB: 0 to 0.5000%, and

the balance being Ni and impurities, wherein

in a section perpendicular to a longitudinal direction of the Ni-basedalloy, an average concentration of Mo is 8.0% or more in mass %; amaximum value of the Mo concentration is 11.0% or less in mass %; andfurther an area fraction of a region in which the Mo concentration isless than 8.0% in mass % is less than 2.0%.

Mo segregation is suppressed in the Ni-based alloy according to thepresent embodiment. Therefore, the Ni-based alloy of the presentembodiment has excellent corrosion resistance.

A Ni-based alloy according to configuration of [7] is the Ni-based alloyaccording to [6], wherein

the chemical composition contains

one or more elements selected from the group consisting of Ca, Nd, and Bby a content that satisfies Formula (2):

(Ca+Nd+B)/S≥2.0  (2)

where, each symbol of element in Formula (2) is substituted by a contentin atomic % (at) of a corresponding element.

In this case, the hot workability of the Ni-base alloy is furtherimproved.

A Ni-based alloy according to configuration of [8] is the Ni-based alloyaccording to [6] and [7], wherein

the grain size number conforming to ASTM E112 is 0.0 or more.

In this case, the hot workability of the Ni-based alloy is furtherimproved.

A Ni-based alloy according to configuration of [9] is the Ni-based alloyaccording to any one of [6] to [8], wherein

a total number of Nb carbonitride whose maximum length is 1 to 100 μm is4.0×10⁻²/μm² or less in the Ni-based alloy.

In this case, the hot workability of the Ni-based alloy is furtherimproved.

Here, in the present description, “Nb carbonitride” is a conceptincluding Nb carbide, Nb nitride, and Nb carbonitride, and indicates aprecipitate whose total content of Nb, C, and N is, in mass %, 90% ormore. Moreover, a maximum length of Nb carbonitride refers to a longeststraight line of those that connect arbitrary two points on an interface(boundary) between Nb carbonitride and the mother phase.

Hereinafter, a method for producing a Ni-based alloy, and a Ni-basedalloy according to the present embodiment will be described.

First Embodiment [Production Method of Ni-Based Alloy]

The method for producing a Ni-based alloy according to the presentembodiment includes a casting step and a segregation reducing step.Hereinafter, each step will be described.

[Casting Step]

In the casting step, a liquid alloy of Ni-based alloy starting materialis melted, and the liquid alloy is cast to produce a Ni-based alloystarting material having the following chemical composition.

[Chemical Composition]

The chemical composition of the Ni-based alloy starting materialcontains the following elements. Hereinafter, “%” concerning an elementmeans, unless otherwise stated, mass %. Note that the chemicalcomposition of a Ni-based alloy which is produced by the productionmethod of a Ni-based alloy of the present embodiment is the same as thechemical composition of the Ni-based alloy starting material.

C: 0.100% or less

Carbon (c) is unavoidably contained. That is, the C content is more than0%. When the C content is too high, carbides typified by Cr carbideprecipitate at grain boundaries as a result of long-time use at a hightemperature. In this case, the corrosion resistance of the Ni-basedalloy will deteriorate. Precipitation of carbides at grain boundariesfurther deteriorates mechanical properties such as toughness of theNi-based alloy. Therefore, the C content is 0.100% or less. The upperlimit of the C content is preferably 0.070%, more preferably 0.050%,further preferably 0.030%, further preferably 0.025%, and furtherpreferably 0.023%. The C content is preferably as low as possible.However, extreme reduction of the C content will increase the productioncost. Therefore, the lower limit of the C content is preferably 0.001%,more preferably 0.005%, and further preferably 0.010%.

Si: 0.50% or less

Silicon (Si) is unavoidably contained. That is, the Si content is morethan 0%. Si deoxidizes a Ni-based alloy. However, when the Si content istoo high, Si combines with Ni or Cr, etc. to form inter metalliccompounds, or to facilitate generation of intermetallic compounds suchas a sigma phase (σ phase). As a result, the hot workability of theNi-based alloy deteriorates. Therefore, the Si content is 0.50% or less.The upper limit of the Si content is preferably 0.40%, more preferably0.30%, further preferably 0.25%, further preferably 0.20%, and furtherpreferably 0.19%. The lower limit of the Si content to effectivelyachieve the above-described deoxidization effects is preferably 0.01%,more preferably 0.02%, and further preferably 0.04%.

Mn: 0.50% or less

Manganese (Mn) is unavoidably contained. That is, the Mn content is morethan 0%. Mn deoxidizes a Ni-based alloy. Mn further immobilizes S, whichis an impurity, as Mn sulfide, thereby improving the hot workability ofthe Ni-based alloy. However, when the Mn content is too high, formationof oxide film of spinel type is facilitated during use in ahigh-temperature corrosion environment, resulting in deterioration ofoxidation resistance at high temperatures. When the Mn content is toohigh, further, the hot workability of the Ni-based alloy deteriorates.Therefore, the Mn content is 0.50% or less. The upper limit of the Mncontent is preferably 0.40%, more preferably 0.30%, and furtherpreferably 0.23%. The lower limit of the Mn content to effectivelyimprove hot workability is preferably 0.01%, more preferably 0.02%,further preferably 0.04%, further preferably 0.08%, and furtherpreferably 0.12%.

P: 0.015% or less

Phosphorus (P) is an impurity. The P content may be 0%. P deterioratesthe toughness of a Ni-based alloy. Therefore, the P content is (0% ormore, and) 0.015% or less. The upper limit of the P content ispreferably 0.013%, more preferably 0.012%, and further preferably0.010%. The P content is preferably as low as possible. However, extremereduction of the P content will increase the production cost. Therefore,the lower limit of the P content is preferably 0.001%, more preferably0.002%, and further preferably 0.004%.

S: 0.0150% or less

Sulfur (S) is an impurity which is unavoidably contained. That is, the Scontent is more than 0%. S deteriorates the hot workability of aNi-based alloy. Therefore, the S content is 0.0150% or less. The upperlimit of the S content is preferably 0.0100%, more preferably 0.0080%,further preferably 0.0050%, further preferably 0.0020%, furtherpreferably 0.0015%, further preferably 0.0010%, and further preferably0.0007%. The S content is preferably as low as possible. However,extreme reduction of the S content will increase the production cost.Therefore, the lower limit of the S content in view point of productioncost is preferably 0.0001%, and more preferably 0.0002%.

Cr: 20.0 to 23.0%

Chromium (Cr) improves the corrosion resistance such as oxidationresistance, water vapor oxidation resistance, and high-temperaturecorrosion resistance of a Ni-based alloy. Further, Cr combines with Nbto form an intermetallic compound and precipitate at grain boundaries,thereby improving the creep strength of a Ni-based alloy. When the Crcontent is too low, the above-described effects cannot be achievedsufficiently. On the other hand, when the Cr content is too high,carbide of M₂₃C₆ type precipitates in a large amount, and thereby thecorrosion resistance rather deteriorates. Therefore, the Cr content is20.0 to 23.0%. The lower limit of the Cr content is preferably 20.5%,more preferably 21.0%, and further preferably 21.2%. The upper limit ofthe Cr content is preferably 22.9%, more preferably 22.5%, furtherpreferably 22.3%, and further preferably 22.0%.

Mo: 8.0 to 10.0%

Molybdenum (Mo) improves the corrosion resistance of a Ni-based alloy inhigh-temperature corrosion environments. Further, Mo dissolves into thematrix, and improves the creep strength of a Ni-based alloy by solidsolution strengthening. As a result, the strength of the Ni-based alloyin a high-temperature corrosion environment increases. On the otherhand, when the Mo content is too high, the hot workability deteriorates.Therefore, the Mo content is 8.0 to 10.0%. The lower limit of the Mocontent is preferably 8.1%, more preferably 8.2%, further preferably8.3%, further preferably 8.4%, and further preferably 8.5%. The upperlimit of the Mo content is preferably 9.9%, more preferably 9.5%,further preferably 9.2%, further preferably 9.0%, and further preferably8.8%.

One or more elements selected from the group consisting of Nb and Ta:3.150 to 4.150%

Niobium (Nb) and Tantalum (Ta) both facilitate generation ofintermetallic compound, thereby contributing to precipitationstrengthening at grain boundaries and within grains. As a result, thecreep strength increases. When the total content of one or more elementsselected from the group consisting of Nb and Ta is too low, theabove-described effects cannot be sufficiently achieved. On the otherhand, when the total content of one or more elements selected from thegroup consisting of Nb and Ta is too high, precipitates become coarse,thereby decreasing the creep strength. Therefore, the total content ofone or more elements selected from the group consisting of Nb and Ta is3.150 to 4.150%. The lower limit of the total content of one or moreelements selected from the group consisting of Nb and Ta is preferably3.200%, more preferably 3.210%, and further preferably 3.220%. The upperlimit of the total content of one or more elements selected from thegroup consisting of Nb and Ta is preferably 4.120%, more preferably4.000%, further preferably 3.800%, further preferably 3.500%, andfurther preferably 3.450%. Note that only Nb may be contained, and Tamay not be contained. Moreover, only Ta may be contained, and Nb may notbe contained. Both Nb and Ta may be contained. When only Nb out of Nband Ta is contained, the above-described total content (3.150 to 4.150%)means the content of Nb. When only Ta out of Nb and Ta is contained, theabove-described total content (3.150 to 4.150%) means the content of Ta.

Ti: 0.05 to 0.40%

Titanium (Ti), along with Si, Mn, and Al, deoxidizes a Ni-based alloy.Further, Ti along with Al forms a gamma prime phase (γ′ phase), therebyimproving the creep strength of a Ni-based alloy under ahigh-temperature corrosive environment. When the Ti content is too low,the above-described effects cannot be sufficiently achieved. On theother hand, when the Ti content is too high, a large amount of carbideand/or oxide is generated, thus deteriorating the hot workability andthe creep strength of a Ni-based alloy. Therefore, the Ti content is0.05 to 0.40%. The lower limit of the Ti content is preferably 0.08%,more preferably 0.10%, further preferably 0.13%, and further preferably0.15%. The upper limit of the Ti content is preferably 0.35%, morepreferably 0.30%, further preferably 0.25%, and further preferably0.22%.

Al: 0.05 to 0.40%

Aluminum (Al), along with Si, Mn, and Ti, deoxidizes a Ni-based alloy.Further, Al, along with Ti, forms a gamma prime phase (γ′ phase),thereby improving the creep strength of the Ni-based alloy under ahigh-temperature corrosive environment. When the Al content is too low,the above-described effects cannot be sufficiently achieved. On theother hand, when the Al content is too high, oxide-based inclusions aregenerated in a large amount, thus deteriorating the hot workability andthe creep strength of a Ni-based alloy. Therefore, the Al content is0.05 to 0.40%. The lower limit of the Al content is preferably 0.06%,more preferably 0.07%, and further preferably 0.08%. The upper limit ofthe Al content is preferably 0.35%, more preferably 0.32%, furtherpreferably 0.30%, and further preferably 0.27%. Note that the Al contentherein means the content of sol. Al (acid soluble Al).

Fe: 0.05 to 5.00%

Iron (Fe) substitutes for Ni. Specifically, Fe improves the hotworkability of a Ni-based alloy. Further, Fe precipitates Laves phase atgrain boundaries, thereby strengthening the grain boundaries. When theFe content is too low, the above-described effects cannot besufficiently achieved. On the other hand, when the Fe content is toohigh, the corrosion resistance of a Ni-based alloy deteriorates.Therefore, the Fe content is 0.05 to 5.00%. The lower limit of the Fecontent is preferably 0.10%, more preferably 0.50%, further preferably1.00%, further preferably 2.00%, and further preferably 2.50%. The upperlimit of the Fe content is preferably 4.70%, more preferably 4.50%,further preferably 4.00%, and further preferably 3.90%.

N: 0.100% or less

Nitrogen (N) is unavoidably contained. That is, the N content is morethan 0%. N stabilizes the austenite in a Ni-based alloy. Further, Nincreases the creep strength of a Ni-based alloy. However, when the Ncontent is too high, the hot workability of the Ni-based alloydeteriorates. Therefore, the N content is 0.100% or less. The upperlimit of the N content is preferably 0.080%, more preferably 0.050%,further preferably 0.030%, and further preferably 0.025%. Extremereduction of the N content will increase the production cost. Therefore,in viewpoint of production cost, the lower limit of the N content ispreferably 0.001%, more preferably 0.002%, and further preferably0.005%.

O: 0.1000% or less

Oxygen (O) is an impurity. The O content may be 0%. O generates oxides,thereby deteriorates the hot workability of a Ni-based alloy. Therefore,the O content is (0% or more, and) 0.1000% or less. The upper limit ofthe O content is preferably 0.0800%, more preferably 0.0500%, furtherpreferably 0.0300%, and further preferably 0.0150%. The O content ispreferably as low as possible. However, extreme reduction of the Ocontent will increase the production cost. Therefore, in viewpoint ofproduction cost, the lower limit of the O content is preferably 0.0001%,more preferably 0.0002%, and further preferably 0.0005%.

The balance of the Ni-based alloy starting material according to thepresent invention is nickel (Ni) and impurities. Note that an impurityherein means an element which is mixed in from ores and scraps as theraw material, or from the environment of production process, etc. whenthe Ni-based alloy is industrially produced.

Note that Ni stabilizes austenite in the structure of a Ni-based alloyand improves the corrosion resistance of the Ni-based alloy. Asdescribed above, the balance other than the above-described elements ofthe chemical composition is Ni and impurities. The lower limit of the Nicontent is preferably 58.0%, more preferably 59.0%, and furtherpreferably 60.0%.

The Ni-based alloy starting material of the present embodiment mayfurther contain, in place of part of Ni, one or more elements selectedfrom the group consisting of Co and Cu. Both of Co and Cu increase thehigh-temperature strength of a Ni-based alloy.

Co: 0 to 1.00%

Cobalt (Co) is an optional element. That is, the Co content may be 0%.When contained, Co increases the high-temperature strength of a Ni-basedalloy. When Co is contained even in a small amount, the above-describedeffects can be achieved to some extent. However, when the Co content istoo high, the hot workability of a Ni-based alloy deteriorates.Therefore, the Co content is 0 to 1.00%. The upper limit of the Cocontent is preferably 0.90%, more preferably 0.80%, further preferably0.70%, and further preferably 0.60%. The lower limit of the Co contentis preferably 0.01%, more preferably 0.10%, further preferably 0.20%,and further preferably 0.30%.

Cu: 0 to 0.50%

Copper (Cu) is an optional element. That is, the Cu content may be 0%.When contained, Cu precipitates to increase the high-temperaturestrength of a Ni-based alloy. When Cu is contained even in a smallamount, the above-described effects can be achieved to some extent.However, when the Cu content is too high, the hot workability of aNi-based alloy deteriorates. Therefore, the Cu content is 0 to 0.50%.The upper limit of the Cu content is preferably 0.45%, more preferably0.40%, further preferably 0.30%, further preferably 0.20%, and furtherpreferably 0.15%. The lower limit of the Cu content is preferably 0.01%,more preferably 0.02%, and further preferably 0.05%.

The Ni-base alloy starting material of the present embodiment mayfurther contain, in place of part of Ni, one or more elements selectedfrom the group consisting of Ca, Nd, and B.

At least one or more elements selected from the group consisting of Ca,Nd, and B: 0 to 0.5000% in total content

All of calcium (Ca), neodymium (Nd), and boron (B) are optionalelements, and may not be contained. That is, the Ca content may be 0%,the Nd content may be 0%, and the B content may be 0%. When at least oneor more elements selected from the group consisting of Ca, Nd, and B arecontained, all of these elements improve the hot workability of aNi-based alloy. Since it is satisfactory that at least one or moreelements selected from the group consisting of Ca, Nd, and B arecontained, for example, only Ca may be contained, only Nd may becontained, and only B may be contained. Ca and Nd may be contained, Caand B may be contained, and Nd and B may be contained. Ca, Nd, and B maybe contained. When at least one or more elements selected from the groupconsisting of Ca, Nd, and B are contained even in a small amount, theabove-described effects can be achieved to some extent. However, Ca, Nd,and B are likely to be absorbed into slag while the liquid alloy ismelted, and are not likely to remain in the Ni-based alloy startingmaterial. For that reason, the total content of Ca, Nd, and B is notlikely to be more than 0.5000%. Therefore, the total content of at leastone or more elements selected from the group consisting of Ca, Nd, and Bis 0 to 0.5000%. The upper limit of the total content of at least one ormore elements selected from the group consisting of Ca, Nd, and B ispreferably 0.4500%, and more preferably 0.4200%. The lower limit of thetotal content of at least one or more elements selected from the groupconsisting of Ca, Nd, and B is preferably 0.0001%, more preferably0.0003%, and further preferably 0.0005%.

A liquid alloy is melted such that the chemical composition of theNi-based alloy starting material has the above-described chemicalcomposition. The liquid alloy may be melted by a well-known method. Theliquid alloy is produced by, for example, electric furnace melting. Theliquid alloy may be melted by vacuum melting. In viewpoint of productioncost, the liquid alloy is preferably melted by electric furnace melting.

The melted liquid alloy is used to produce a Ni-based alloy startingmaterial having the above-described chemical composition by a castingmethod. The Ni-base alloy starting material may be an ingot produced byan ingot-making process, or a cast piece (slab or bloom) produced by acontinuous casting process.

A solidification cooling rate V_(R) from the state of a liquid alloyuntil the solidified state as a Ni-based alloy starting material in thecasting step can be calculated by measuring dendrite secondary armspacing D_(II) of the Ni-based alloy starting material after castingstep and before the segregation reducing step. The dendrite secondaryarm spacing D_(II) can be measured by the following method. A sample iscollected at a W/4 depth position of a section perpendicular to thelongitudinal direction (cross section) at a central position in thelongitudinal direction of the Ni-based alloy starting material. Aftermirror polishing is performed on a surface parallel with theabove-described cross section out of the surfaces of the sample, etchingby aqua regia is performed. The etched surface is observed by an opticalmicroscope of 400 times magnification to generate a photographic imageof an observation field of view of 200 μm×200 μm. Using the obtainedphotographic image, dendrite secondary arm spacing (μm) at arbitrary 20locations in the observation field of view are measured. An average ofthe measured dendrite secondary arm spacing is defined as a dendritesecondary arm spacing D_(II) (μm).

A solidification cooling rate V_(R) (° C./min) is determined bysubstituting the determined dendrite secondary arm spacing D_(II) forFormula (A).

D_(II)=182V_(R) ^(−0.294)  (A)

[Segregation Reducing Step]

In the segregation reducing step, Mo segregation is reduced for theNi-base alloy starting material produced in the casting step.Specifically, for the Ni-based alloy starting material produced in thecasting step:

(I) heat treatment, or

(II) heat treatment, and complex treatment after the heat treatment areperformed.

In the present description, “complex treatment” means a series oftreatments in which hot working is performed, and further, heattreatment is performed after the hot working. In other words, “complextreatment” means a combined treatment of hot working at one time andheat treatment at one time after the hot working. Heat treatment at onetime means a treatment in which an object is inserted into a reheatingfurnace or a soaking pit and is retained at a predetermined holdingtemperature for a predetermined holding time, thereafter beingextracted. Hot working at one time means a treatment starting from hotworking on a Ni-based alloy starting material heated to 1000 to 1300° C.ending in the hot working. Hot working means, for example, hotextrusion, hot forging, and hot rolling.

In the segregation reducing step, the heat treatment may be performedonly at one time without performing the complex treatment, or thecomplex treatment may be performed only at one time without performingthe heat treatment. Moreover, the complex treatment may be performedrepeatedly at multiple times. The complex treatment at one or more timesmay be performed after the heat treatment at one or more times. The heattreatment at one or more times may be performed after the complextreatment at one or more times. In short, in the segregation reducingstep, the heat treatment at least one time, or the heat treatment atleast one time and the complex treatment at least one time may beperformed.

After heat treatment, the complex treatment may be performed in the samestatus, or after heat treatment, the Ni-based alloy starting materialmay be once cooled, and the heat treatment may be performed again,thereafter performing the complex treatment (that is, in this case, heattreatment, heat treatment, and complex treatment are performed in thisorder). Moreover, the complex treatment may be performed after the heattreatment, and thereafter, the complex treatment may be performed (inthis case, the heat treatment, the complex treatment, and the complextreatment are performed in this order). The heat treatment and thecomplex treatment may be appropriately combined. For example, theperforming order may be in the order of heat treatment, complextreatment, and heat treatment, or in the order of heat treatment,complex treatment, heat treatment, and complex treatment.

Hereinafter, the hot working during the heat treatment and the complextreatment will be described.

[Heat Treatment]

In the n-th heat treatment, the Ni-based alloy starting materialproduced by the casting step is retained at a holding temperature T_(n)(° C.) for a holding time t_(n) (hr). Where, n is 1 to N (N is a naturalnumber), the holding temperature T_(n) means the holding temperature (°C.) of the n-th heat treatment (including the heat treatment of theabove-described (I) and the heat treatment of the above-described (II)),the holding time t_(n) means the holding time (hr) of the n-th heattreatment. N is a total number of the heat treatment of theabove-described (I) and the heat treatment of the above-described (II).

When the holding temperature T_(n) is too low, the diffusion distance σof Mo cannot be increased, and Mo is not likely to diffuse during theheat treatment. On the other hand, when the holding temperature T_(n) istoo high, part of the Ni-based alloy starting material may possibly beremelted. Therefore, although the holding temperature T_(n) is notparticularly limited, the holding temperature T_(n) is preferably 1000to 1300° C. The heat treatment can be sufficiently performed by awell-known reheating furnace or a soaking pit.

[Hot Working]

The hot working may be, as described above, hot extrusion, hot forging,and hot rolling. The types of hot working will not be particularlylimited. In the production method of the present embodiment, when hotworking is performed, the above-described heat treatment is performedafter the hot working (complex treatment). Owing to the hot working, theMo inter-segregation distance Ds in the Ni-based alloy starting materialhas been decreased. For that reason, in the heat treatment after the hotworking, Mo is more likely to diffuse, thereby reducing the holding timeto which is needed for reducing Mo segregation. Note that in thesegregation reducing step, when the complex treatment is performedwithout the heat treatment being performed in a preceding stage, theNi-based alloy starting material is heated to 1000 to 1300° C. in areheating furnace of a soaking pit, and is thereafter subjected to hotworking.

[Formula (1)]

As described above, in the segregation reducing step, heat treatment atone or more times, or heat treatment at one or more times and complextreatment at one or more times are performed. In this occasion, theholding temperature T_(n) (° C.), the holding time t_(n) (hr), and thearea reduction ratio Rd_(n-1)(%) are adjusted such that Formula (1) issatisfied.

     [Expression  7] $\begin{matrix}{V_{R}^{- 0.294} \leq {1.27 \times 10^{3}\mspace{14mu} {\sum\limits_{n = 1}^{N}\; \sqrt{\left( {1 - \frac{{Rd}_{n - 1}}{100}} \right)^{- 1} \cdot {\exp \left( \frac{{- 2.89} \times 10^{4}}{T_{n} + 273} \right)} \cdot t_{n}}}}} & (1)\end{matrix}$

Note that when the heat treatment is performed only at one time, and thecomplex treatment is not performed in the segregation reducing step(that is, when n=1, and N=1), hot working will not be performed in thesegregation reducing step. For that reason, the cumulative areareduction ratio Rd_(n-1)=Rd₀ will be 0(%). Therefore, based on thefollowing Formula which is obtained by substituting Rd₀=0 for Formula(1), the solidification cooling rate V_(R) (° C./min), the holdingtemperature T_(n) (° C.), and the holding time t_(n) (hr) are adjusted.

     [Expression  8]$V_{R}^{- 0.294} \leq {1.27 \times 10^{3}\mspace{14mu} {\sum\limits_{n = 1}^{N}\; \sqrt{{\exp \left( \frac{{- 2.89} \times 10^{4}}{T_{n} + 273} \right)} \cdot t_{n}}}}$

If the segregation reducing step (the heat treatment, or the heattreatment and the complex treatment) is performed so as to satisfyFormula (1), it is possible to produce a Ni-based alloy in which Mosegregation is suppressed. Note that after the segregation reducing stepis performed, other steps such as a hot working step, a cold workingstep, and a cutting step may be performed.

[Ni-Based Alloy According to the Present Embodiment]

The shape of the Ni-based alloy according to the present embodiment willnot be particularly limited. The Ni-based alloy produced by theabove-described production method is, for example, a billet. The section(cross section) perpendicular to the longitudinal direction of theNi-based alloy may be of a circular shape, a rectangular shape, or apolygonal shape. The Ni-based alloy may be a pipe, or a solid material.

The Ni-based alloy according to the present invention has a chemicalcomposition consisting of: in mass %, C: 0.100% or less, Si: 0.50% orless, Mn: 0.50% or less, P: 0.015% or less, S: 0.0150% or less, Cr: 20.0to 23.0%, Mo: 8.0 to 10.0%, one or more elements selected from the groupconsisting of Nb and Ta: 3.150 to 4.150%, Ti: 0.05 to 0.40%, Al: 0.05 to0.40%, Fe: 0.05 to 5.00%, N: 0.100% or less, O: 0.1000% or less, Co: 0to 1.00%, Cu: 0 to 0.50%, one or more elements selected from the groupconsisting of Ca, Nd, and B: 0 to 0.5000%, and the balance being Ni andimpurities. That is, the chemical composition of the Ni-based alloy ofthe present embodiment is the same as the chemical composition of theabove-described Ni-based alloy starting material. Further in theNi-based alloy of the present embodiment, in a section perpendicular tothe longitudinal direction of the Ni-based alloy, an averageconcentration of Mo is 8.0% or more in mass %, a maximum value of Moconcentration is 11.0% or less in mass %, and further an area ratio of aregion in which Mo concentration is less than 8.0% in mass % is lessthan 2.0%. In the Ni-based alloy according to the present embodiment,segregation of Mo is suppressed. Hereinafter, the Ni-based alloy of thepresent embodiment will be described. Note that the content (including apreferable upper limit and a preferable lower limit) of each element ofthe chemical composition and advantageous effects of the Ni-based alloyof the present embodiment are the same as the content (including apreferable upper limit and a preferable lower limit) of each element ofthe chemical composition and the advantageous effects of the Ni-basedalloy starting material in the above-described production method of aNi-based alloy.

[Suppression of Mo Segregation]

In the Ni-based alloy of the present embodiment, Mo segregation issuppressed. Specifically, in a section perpendicular to the longitudinaldirection of the Ni-based alloy (hereinafter, referred to as a crosssection), an average concentration of Mo is 8.0% or more in mass %, amaximum value of Mo concentration is 11.0% or less in mass %, andfurther an area fraction of a region in which Mo concentration is lessthan 8.0% in mass % is less than 2.0%.

The average concentration of Mo, the maximum value of Mo concentration,and the region in which the Mo concentration is less than 8.0% in mass %in a cross section of the Ni-based alloy are determined by the followingmethod. Note that, in the present description, a region in which Moconcentration is less than 8.0% in mass % is also referred to as a “Molow-concentration region”.

A sample is collected from a cross section of Ni-based alloy.Specifically, when the Ni-based alloy is a solid material whose crosssectional shape is a rectangular shape, the long side of the crosssection is defined as a width W. When it is a solid material (that is,bar blank) whose cross section is of a circular shape, the diameter isdefined as a width W. When the Ni-based alloy is a solid material, asample is collected from a W/4 depth position in the width W directionfrom a surface perpendicular to the width W direction (W/4 depthposition). On the other hand, when the Ni-based alloy is a pipe, asample is collected from a wall-thickness central position. Out of thesurface of the sample, a surface (observation surface) corresponding tothe cross section is mirror polished, and line analysis by an electronprobe micro analyzer (EPMA) is performed with a beam diameter: 10 μm, ascanning length: 2000 μm, an irradiation time for one point: 3000 ms,and an irradiation pitch: 5 μm in any one field of view in theobservation surface. In the scanning rage of 2000 μm in which the lineanalysis has been performed, an average value of multiple Moconcentrations measured at a 5 μm pitch, a maximum value of Moconcentration and a minimum value of Mo concentration of the multiplemeasured Mo concentrations are determined. Further, in the scanninglength 2000 μm which is the measurement range, a total length of rangesin which measured points at which Mo concentration has turned out to beless than 8.0% are continuous (a range in which two or more points arecontinuous) is determined. The determined total length is defined astotal length of Mo low-concentration region (μm). The determined totallength of Mo low-concentration region is used to define a fraction of Molow-concentration region (%) according to the following formula.

Fraction of Mo low-concentration region=total length of Molow-concentration region (μm)/scanning length(=2000 μm)×100

The fraction of Mo low-concentration region determined by the abovedescribed formula is defined as an “area fraction of region in which Moconcentration is less than 8.0% in mass %”. More specifically, uponperforming line analysis by EPMA with a beam diameter: 10 μm, a scanninglength: 2000 μm, an irradiation time per one point: 3000 ms, and anirradiation pitch: 5 μm, in a cross section of the Ni-based alloy, theaverage concentration of Mo obtained at a pitch of 5 μm in a scanninglength of 2000 μm is 8.0% or more in mass %; the maximum value of Moconcentration is 11.0% or less in mass %; and when a total length ofranges in which measured points, at which the Mo concentration is lessthan 8.0%, in a scanning length of 2000 μm, are continuous (ranges inwhich two or more points are continuous) is defined as an Molow-concentration region, the fraction of the total length of Molow-concentration region with respect to the scanning length is lessthan 2.0%.

In the Ni-based alloy of the present embodiment, an average value of Moconcentration obtained by the above-described measurement is 8.0% ormore in mass %, and a maximum value of Mo concentration is 11.0% or lessin mass %. Further, ratio of region in which Mo concentration is lessthan 8.0% in mass %, that is, the fraction of Mo low-concentrationregion is less than 2.0%.

As described so far, in the Ni-based alloy of the present embodiment, Mosegregation is suppressed. As a result, the corrosion resistance of theNi-based alloy is improved. Specifically, it is possible to suppressintergranular corrosion and stress corrosion cracking, in the followingway.

[Reduction of Intergranular Corrosion]

In the Ni-based alloy according to the present embodiment, when acorrosion test specified by ASTM G28 Method A is performed, a corrosionrate is 0.075 mm/month or less. The corrosion test conforming to ASTMG28 Method A is performed by the following method. A test specimen iscollected from any position of the Ni-based alloy. The size of the testspecimen is, for example, 40 mm×10 mm×3 mm. The weight of the testspecimen before starting corrosion test is measured. After themeasurement, the test specimen is immersed in a solution (50% sulfuricacid/ferric sulfate solution), in which 25 g of ferric sulfate is addedto 600 mL of sulfuric acid solution of 50% in mass %, for 120 hours.After elapse of 120 hours, the weight of the test specimen after testingis measured. Based on the change in the weight of the measured testspecimen, specimen loss due to testing is determined. By use of thedensity of the test specimen, the specimen loss due to testing isconverted into an amount of volume decrease. A corrosion depth isdetermined by dividing the amount of volume decrease by the surface areaof the test specimen. A corrosion rate (mm/month) is determined bydividing the corrosion depth by the test time.

In the Ni-based alloy of the present embodiment, the corrosion rate is0.075 mm/month or less, and thus intergranular corrosion is suppressed,thus exhibiting excellent corrosion resistance.

[Suppression of Stress Corrosion Cracking]

The Ni-based alloy of the present embodiment not only excels inintergranular corrosion resistance, but also is able to suppress stresscorrosion cracking. Specifically, a slow-strain-rate tensile testspecimen is collected from an arbitrary position of the Ni-based alloy.The length of the slow-strain-rate tensile test specimen is 80 mm, thelength of a parallel part is 25.4 mm, and the diameter of the parallelpart is 3.81 mm. The longitudinal direction of the slow-strain-ratetensile test specimen was made parallel with the longitudinal directionof the Ni-based alloy. The slow strain rate tensile test (SSRT) isperformed at a strain rate of 4.0×10⁻⁶ S⁻¹ while immersing theslow-strain-rate tensile test specimen in a water solution of 25%NaCl+0.5% CH₃COOH of pH 2.8 to 3.1 and 232° C., which is saturated with0.7 MPa of hydrogen sulfide, to cause the test specimen to be torn off.In the test specimen after the test, whether or not any sub-crack hasoccurred in a portion other than the torn-off part is visuallyconfirmed. When any sub-crack has occurred, it is judged that stresscorrosion cracking has occurred, and when no sub-crack is confirmed, itis judged that no stress corrosion cracking has occurred. In theNi-based alloy produced by the present production method, no sub-crackis confirmed in the above-described slow strain rate tensile test, andthus stress corrosion cracking is suppressed. Therefore, the Ni-basedalloy produced by the production method of the present embodiment hasexcellent corrosion resistance.

As so far described, in the Ni-based alloy produced by the productionmethod of the present embodiment, the above-described chemicalcomposition is contained, and further an average concentration of Mo is8.0% or more in mass %, a maximum value of Mo concentration is 11.0% orless in mass %. Further, an area fraction of region (Molow-concentration region) in which Mo concentration is less than 8.0% inmass % is less than 2.0%. Therefore, the Ni-base alloy of the presentembodiment is excellent in corrosion resistance. Specifically, acorrosion rate obtained by the ASTM G28 Method A test is 0.075 mm/monthor less, thus exhibiting excellent corrosion resistance (intergranularcorrosion resistance). Further, in the SSRT test, no sub-crack hasoccurred in any region other than the torn-off part of the testspecimen, thus exhibiting excellent corrosion resistance (specifically,SCC resistance).

[Production Method of Ni-Based Alloy of the Present Embodiment]

The production method of a Ni-base alloy of the present embodiment willnot be particularly limited provided that a Ni-based alloy having theabove-described configuration can be produced. However, theabove-described production method of a Ni-based alloy is a suitableexample for producing a Ni-base alloy of the present embodiment.Specifically, the production method of a Ni-base alloy of the presentembodiment includes the above-described casting step and theabove-described segregation reducing step. In the above-describedcasting step, liquid alloy is cast to produce a Ni-based alloy startingmaterial having a chemical composition consisting of: in mass %, C:0.100% or less, Si: 0.50% or less, Mn: 0.50% or less, P: 0.015% or less,S: 0.0150% or less, Cr: 20.0 to 23.0%, Mo: 8.0 to 10.0%, one or moreelements selected from the group consisting of Nb and Ta: 3.150 to4.150%, Ti: 0.05 to 0.40%, Al: 0.05 to 0.40%, Fe: 0.05 to 5.00%, N:0.100% or less, O: 0.1000% or less, Co: 0 to 1.00%, Cu: 0 to 0.50%, oneor more elements selected from the group consisting of Ca, Nd, and B: 0to 0.5000%, and the balance being Ni and impurities. Then, in thesegregation reducing step, (I) heat treatment at one or more times, or(II) heat treatment at one or more times and complex treatment at one ormore times are performed on the Ni-base alloy starting material producedby the casting step to satisfy Formula (1).

     [Expression  9] $\begin{matrix}{V_{R}^{- 0.294} \leq {1.27 \times 10^{3}\mspace{14mu} {\sum\limits_{n = 1}^{N}\; \sqrt{\left( {1 - \frac{{Rd}_{n - 1}}{100}} \right)^{- 1} \cdot {\exp \left( \frac{{- 2.89} \times 10^{4}}{T_{n} + 273} \right)} \cdot t_{n}}}}} & (1)\end{matrix}$

By the above-described production method, a Ni-based alloy having achemical composition consisting of: in mass %, C: 0.100% or less, Si:0.50% or less, Mn: 0.50% or less, P: 0.015% or less, S: 0.0150% or less,Cr: 20.0 to 23.0%, Mo: 8.0 to 10.0%, one or more elements selected fromthe group consisting of Nb and Ta: 3.150 to 4.150%, Ti: 0.05 to 0.40%,Al: 0.05 to 0.40%, Fe: 0.05 to 5.00%, N: 0.100% or less, O: 0.1000% orless, Co: 0 to 1.00%, Cu: 0 to 0.50%, one or more elements selected fromthe group consisting of Ca, Nd, and B: 0 to 0.5000%, and the balancebeing Ni and impurities, wherein, in a section perpendicular to thelongitudinal direction of the Ni-based alloy, an average concentrationof Mo is 8.0% or more in mass %, a maximum value of Mo concentration is11.0% or less in mass %, and further an area ratio of a region in whichMo concentration is less than 8.0% in mass % is less than 2.0% can beproduced.

FIG. 4 is a diagram to show relationship between F1 and the corrosionrate in a Ni-based alloy having the chemical composition of the presentinvention. Where, F1 is an expression obtained by subtracting the lefthand side of Formula (1) from the right hand side of Formula (1), and isdefined as follows.

     [Expression  10]${F\; 1} = {{1.27 \times 10^{3}\mspace{14mu} {\sum\limits_{n = 1}^{N}\; \sqrt{\left( {1 - \frac{{Rd}_{n - 1}}{100}} \right)^{- 1} \cdot {\exp \left( \frac{{- 2.89} \times 10^{4}}{T_{n} + 273} \right)} \cdot t_{n}}}} - V_{R}^{- 0.294}}$

Referring to FIG. 4, when F1 is less than 0, that is the productioncondition in the segregation reducing step does not satisfy Formula (1),the corrosion rate is remarkably higher than 0.075 mm/month, and thecorrosion rate will not vary significantly even when F1 value varies. Incontrast to this, when F1 is 0 or more, that is, the productioncondition in the segregation reducing step satisfies Formula (1), thecorrosion rate remarkably decreases to be 0.075 mm/month or less.Therefore, a Ni-base alloy produced in a production condition thatsatisfies Formula (1) has excellent corrosion resistance. Note that theproduction method of a Ni-based alloy of the present embodiment will notbe particularly limited provided that a Ni-based alloy having theabove-described configuration can be produced. The above-describedproduction method using Formula (1) is a suitable example for producinga Ni-based alloy of the present embodiment.

[Preferable Form (1) of Ni-Based Alloy of First Embodiment]

It is known that in a Ni-based alloy, the finer the crystal grains, themore excellent the strength and toughness will be. Preferably, aNi-based alloy of the present embodiment has a grain size numberconforming to ASTM E112 of 0.0 or more. A grain size number of 0.0 ormore indicates that solidification structure is dissolved and themicrostructure is substantially crystallized in the Ni-based alloy. Thegrain size number is preferably 0.5 or more, and more preferably 1.0 ormore. The upper limit of grain size number will not be particularlylimited.

The measurement method of grain size number in a Ni-based alloy of thepresent embodiment is as follows. A Ni-based alloy is divided into 5equal sections in the axial direction (longitudinal direction) and anaxially central position of each section is identified. At theidentified position of each section, four sample collection positionsare identified at a pitch of 90° around the central axis of the Ni-basedalloy. For example, when the Ni-based alloy is a pipe, sample collectionpositions are identified at a 90 degree pitch in the pipecircumferential direction. Samples are collected from the identifiedsample collection positions. When the Ni-based alloy is a pipe, a sampleis collected from the wall-thickness central position of each of theidentified sample collection positions. When the Ni-based alloy is abar, or an alloy having a cross section of a rectangular shape, a sampleis collected from a W/4 depth position in a selected sample collectionposition. It is supposed that the observation surface of sample is asection perpendicular to the axial direction of the Ni-based alloy, andthe area of the observation surface is 40 mm².

According to the above-described method, four samples in each section,and 20 samples in all the sections are collected. Each observationsurface of the collected samples is etched by using Glyceregia,Kalling's reagent, or Marble's reagent, etc. to cause grain boundariesin the surface to appear. The etched observation surface is observed todetermine the grain size number in conformity with ASTM E112.

An average value of the grain size numbers determined in the 20 samplesis defined as the grain size number conforming to ASTM E112 in theNi-based alloy.

A Ni-based alloy, which is the Ni-based alloy of the present embodiment,and whose grain size number conforming to ASTM E112 is 0.0 or more, isproduced, for example, by the following method.

In the production method of Ni-based alloy including the above-describedcasting step and segregation reducing step, a complex treatment isperformed at least one time in the segregation reducing step. Then, inthe complex treatment, hot working at an area reduction ratio of 35.0%or more is performed at least one time for the Ni-base alloy startingmaterial which has been heated to 1000 to 1300° C. The hot working inthis condition is referred to as “specific hot working”. In thesegregation reducing step, when the specific hot working is performed atleast one time, the grain size number conforming to ASTM E112 will be0.0 or more in the produced Ni-based alloy. Note that the area reductionratio herein does not mean an cumulative area reduction ratio, but meansan area reduction ratio in hot working at one time.

FIG. 5A is a microstructure observation image of a Ni-based alloyproduced by performing hot working one time at an area reduction ratioof 44.6% for a Ni-based alloy starting material having theabove-described chemical composition in the segregation reducing step.FIG. 5B is a microstructure observation image of a Ni-based alloyproduced by performing hot working one time at an area reduction ratioof 31.3% for the Ni-based alloy starting material having theabove-described chemical composition in the segregation reducing step.In FIG. 5A, the grain size number conforming to ASTM E112 was 2.0, thatis, 0.0 or more. In contrast to this, in FIG. 5B, the grain size numberconforming to ASTM E112 was −2.0, that is, less than 0.0. As describedso far, in the segregation reducing step, by performing hot working atan area reduction ratio of 35.0% or more at least one time for aNi-based alloy starting material having the above-described chemicalcomposition, it is possible to produce a Ni-based alloy having a grainsize number conforming to ASTM E112 of 0.0 or more. Note that thespecific hot working may be performed multiple times.

[Preferable Form (2) of Ni-Based Alloy of First Embodiment]

Preferably, in the Ni-based alloy of the present embodiment, further,the total number of Nb carbonitride whose maximum length is 1 to 100 μmis 4.0×10⁻² μm² or less in the Ni-based alloy.

Where, “Nb carbonitride” herein is a concept including Nb carbide, Nbnitride, and Nb carbonitride, and means a precipitate in which a totalcontent of Nb, C, and N is, in mass %, 90% or more. Moreover, themaximum length of Nb carbonitride means the maximum length of straightlines connecting arbitrary two points on the interface (boundary)between Nb carbonitride and the mother phase.

When the total number of coarse Nb carbonitride is 4.0×10⁻²/μm² or less,Nb carbonitride is sufficiently dissolved into the matrix. For thatreason, starting points of cracking during hot working decrease, andthus hot workability is further improved.

The total number of coarse Nb carbonitride can be determined by thefollowing method. The Ni-based alloy is divided into 5 equal sections inthe axial direction, and an axially central position of each section isidentified. In each section, sample collection positions are identifiedat 90 degree pitch in the pipe circumferential direction at the axiallycentral position. Samples are collected from the identified samplecollection positions. When Ni-based alloy is a pipe, a sample iscollected from the wall-thickness central position of each of theidentified sample collection positions. When the Ni-based alloy is abar, or an alloy having a cross section of a rectangular shape, a sampleis collected from a W/4 depth position at an identified samplecollection position. The observation surface of sample is a sectionperpendicular to the axial direction of the Ni-based alloy. In any onefield of view (400 μm×400 μm) in each observation surface (of a total of20), Nb carbonitride is identified by EPMA (Electron Probe MicroAnalyzer). Specifically, a precipitate in which a total content of Nb,C, and N is 90% or more is identified by plane analysis of EPMA, and theidentified precipitate is defined as Nb carbonitride. FIG. 6 is an EPMAimage in one example of the above-described one field of view. Aprecipitate 100 which is displayed in white in FIG. 6 is Nbcarbonitride. A maximum length of the identified Nb carbonitride ismeasured. As described so far, among straight lines connecting arbitrarytwo points on the interface between Nb carbonitride and the motherphase, the value of the longest straight line is defined as the maximumlength of the Nb carbonitride. After measuring the maximum length ofeach Nb carbonitride, Nb carbonitride whose maximum length is 1 to 100μm (coarse Nb carbonitride) is identified, and a total number of coarseNb carbonitride in all the 20 fields of view is determined. Based on theobtained total number, a total number of coarse Nb carbonitride (1/μm²)is determined.

A Ni-based alloy, which is the above-described Ni-based alloy, and inwhich a total number of Nb carbonitride whose maximum length is 1 to 100μm is 4.0×10⁻²/μm² or less can be produced by the following productionmethod.

In a production method of a Ni-based alloy, including theabove-described casting step and the segregation reducing step, heattreatment in which the holding temperature is 1000 to 1300° C., and theholding time is 1.0 hour or more is performed at least one time in thesegregation reducing step. The heat treatment in this condition isreferred to as “specific heat treatment”. When the specific heattreatment is performed at least one time in the segregation reducingstep, a total number of Nb carbonitride whose maximum length is 1 to 100μm will be 4.0×10⁻²/μm² or less. Note that the specific heat treatmentmay be performed multiple times.

[Preferable Form (3) of Ni-Based Alloy of First Embodiment]

The above-described Ni-based alloy may further have a grain size numberconforming to ASTM E112 of 0.0 or more, and a total number of Nbcarbonitride whose maximum length is 1 to 100 μm will be 4.0×10⁻²/μm² orless in the Ni-based alloy.

In this case, preferably, in the above-described segregation reducingstep, hot working at an area reduction ratio of 35.0% or more isperformed at least one time for the Ni-base alloy starting materialwhich has been heated to 1000 to 1300° C., and also in theabove-described segregation reducing step, heat treatment in which theholding temperature is 1000 to 1300° C., and the holding time is 1.0hour or more is performed at least one time. That is, in the segregationreducing step, the specific hot working is performed at least one time,and the specific heat treatment is performed at least one time.

Second Embodiment

Preferably, the above-described Ni-based alloy further contains one ormore elements selected from the group consisting of Ca, Nd, and B by acontent to satisfy Formula (2):

(Ca+Nd+B)/S≥2.0  (2)

where, each symbol of element in Formula (2) is substituted by a contentin atomic % (at %) of a corresponding element.

All of calcium (Ca), neodymium (Nd), and boron (B) improve hotworkability of a Ni-based alloy as described above. Definition is madeas F2=(Ca+Nd+B)/S. F2 is an index of hot workability. When a totalcontent F2 of one or more elements selected from the group consisting ofCa, Nd, and B is 2.0 or more, that is, F2 satisfies Formula (2), furtherexcellent hot workability can be achieved in the Ni-based alloy of theabove-described chemical composition. Specifically, reduction (reductionarea after fraction) when tensile test is performed at a strain rate of10/sec, at 900° C. in the atmosphere will be 35.0% or more.

FIG. 7 is a diagram to show relationship between reduction area afterfraction (%), which is obtained when tensile test is performed at astrain rate of 10/sec at 900° C. in the atmosphere for the Ni-basedalloy of the present embodiment, and F2. FIG. 7 is obtained by a testshown in Example 2 to be described below. Referring to FIG. 7, until F2became 1.0, the reduction area after fraction at 900° C. did not varysignificantly even when F2 increased. On the other hand, when F2 becamemore than 1.0, the reduction area after fraction at 900° C. rapidlyincreased as F2 increased, and became more than 35.0% when F2 was 2.0,reaching about 50.0%. Thereafter, although the reduction area afterfraction further increased as F2 increased, the reduction area afterfraction became substantially constant at about 80.0% when F2 was 8.0 ormore. That is, the curve of FIG. 7 had an inflection point in thevicinity of F2=1.0 to 2.0. From the result described so far, if F2 is2.0 or more, it is possible to obtain a sufficient reduction area afterfraction (35.% or more) at 900° C. The lower limit of F2 is preferably2.5, more preferably 3.0, and further preferably 3.5.

Note that the upper limit of the total content (mass %) of Ca, Nd, and Bin a Ni-based alloy is 0.5000% as in the first embodiment.

[Production Method of Ni-Based Alloy of Second Embodiment]

The production method of a Ni-base alloy of the second embodimentdescribed above will not be particularly limited provided that aNi-based alloy having the above-described configuration can be produced.Preferably, the production method of a Ni-based alloy of the secondembodiment is the same as the production method of a Ni-based alloy ofthe first embodiment.

Specifically, the production method of a Ni-based alloy of the secondembodiment includes a casting step and a segregation reducing step. Inthe casting step, a liquid alloy is cast to produce a Ni-based alloystarting material which has the above-described chemical composition andin which F2 satisfies Formula (2).

In the segregation reducing step,

(I) heat treatment, or

(II) heat treatment and complex treatment

are performed on the Ni-based alloy starting material produced in thecasting step. In the segregation reducing step, the heat treatment maybe performed only one time, or the complex treatment may be performedonly one time. Moreover, the complex treatment may be performed multipletimes repeatedly. The complex treatment may be performed after the heattreatment.

As described so far, in the segregation reducing step, the heattreatment, or the heat treatment and the complex treatment areperformed. In this occasion, the holding temperature T_(n) (° C.), theholding time t_(n) (hr), and the area reduction ratio Rd_(n-1)(%) areadjusted such that the solidification cooling rate V_(R) in the castingstep satisfies Formula (1).

     [Expression  11] $\begin{matrix}{V_{R}^{- 0.294} \leq {1.27 \times 10^{3}\mspace{14mu} {\sum\limits_{n = 1}^{N}\; \sqrt{\left( {1 - \frac{{Rd}_{n - 1}}{100}} \right)^{- 1} \cdot {\exp \left( \frac{{- 2.89} \times 10^{4}}{T_{n} + 273} \right)} \cdot t_{n}}}}} & (1)\end{matrix}$

Note that when the heat treatment is performed only one time in thesegregation reducing step, the area reduction ratio R_(d0) is 0(%) sincehot working is not performed. Therefore, based on a formula obtained bysubstituting R_(d0)=0% for Formula (1), the solidification cooling rateV_(R) (° C./min), the holding temperature T_(n) (° C.), and the holdingtime t_(n) (hr) are adjusted.

[Expression  12]$V_{R}^{- 0.294} \leq {1.27 \times 10^{3}\mspace{14mu} {\sum\limits_{n = 1}^{N}\; \sqrt{{\exp \left( \frac{{- 2.89} \times 10^{4}}{T_{n} + 273} \right)} \cdot t_{n}}}}$

Performing the segregation reducing step (heat treatment, or heattreatment and complex treatment) so as to satisfy Formula (1) for theNi-based alloy starting material having the chemical composition thatsatisfies Formula (2) will make it possible to produce a Ni-based alloyof the second embodiment. Note that after the segregation reducing stepis performed, further, other steps such as a hot working step, a coldworking step, and a cutting step may be performed.

Note that the production method of a Ni-based alloy of the secondembodiment does not perform a so-called secondary melting, in whichafter the Ni-based alloy starting material is produced in the castingstep, the Ni-based alloy starting material is remelted. That is, in thepresent production method, it is preferable to perform the segregationreducing step without performing the secondary melting in which theNi-based alloy produced by the casting step is remelted after thecasting step.

In the Ni-based alloy of the second embodiment, Ca, Nd, and B, etc.generally combine with S in a steel material to form sulfide, andimprove hot workability by reducing solid-solution S concentration inthe Ni-based alloy (particularly, at grain boundaries). However, if thesecondary melting is performed on the Ni-based alloy starting materialthat contains these elements, Ca, Nd, and B are discharged from theNi-based alloy starting material to the outside at the time of secondarymelting. For example, if electro slag remelting (ESR) is applied as thesecondary melting, Ca, Nd, and B are taken into a molten slag when theNi-based alloy starting material melts. As a result, Ca, Nd, and B aredischarged from the Ni-based alloy starting material so that thechemical composition of the Ni-based alloy starting material after thesecondary melting will not satisfy Formula (2). Similarly, if the vacuumarc remelting (VAR) is applied as the secondary melting, Ca, Nd, and B,which are effective elements to improve hot workability, will be causedto float to be separated by CO bubbles generated at the time of meltingof the Ni-based alloy starting material. As a result, Ca, Nd, and B aredischarged from the Ni-based alloy starting material, and the chemicalcomposition of the Ni-based alloy starting material produced after thesecondary melting will not satisfy Formula (2). In contrast to this, inthe present production method, as described above, the Ni-based alloystarting material is produced by primary melting alone withoutperforming the secondary melting (omitting the secondary melting). Forthat reason, in the Ni-based alloy, it is possible to maintain one ormore elements of Ca, Nd, and B in a content that satisfies Formula (2),thus improving hot workability. Further, since the above-describedsegregation reducing step is performed on the Ni-based alloy startingmaterial, it is possible to suppress Mo segregation.

[Preferable Form (1) of Ni-Based Alloy of Second Embodiment]

As in the first embodiment, preferably, the grain size number conformingto ASTM E112 is 0.0 or more in the Ni-based alloy of the secondembodiment.

For obtaining a grain size number of 0.0 or more in a Ni-based alloy,preferably, hot working (specific hot working) at an area reductionratio of 35.0% or more is performed at least one time for the Ni-basedalloy starting material which has been heated to 1000 to 1300° C. in theabove-described segregation reducing step. Performing the specific hotworking at least one time in the segregation reducing step will resultin that the grain size number conforming to ASTM E112 will be 0.0 ormore in the produced Ni-based alloy. Note that, the specific hot workingmay be performed multiple times.

[Preferable Form (2) of Ni-Based Alloy of Second Embodiment]

As in the first embodiment, preferably, in the Ni-based alloy of thesecond embodiment, a total number of Nb carbonitride whose maximumlength is 1 to 100 μm is 4.0×10⁻²/μm² or less in the Ni-based alloy. Inthis case, hot workability is further improved.

When making the total number of Nb carbonitride whose maximum length is1 to 100 μm is 4.0×10⁻²/μm² or less in the Ni-based alloy, preferably,heat treatment (specific heat treatment) in which the holdingtemperature is 1000 to 1300° C., and the holding time is 1.0 hour ormore is performed at least one time in the segregation reducing step.Performing the specific heat treatment at least one time will result inthat the total number of Nb carbonitride whose maximum length is 1 to100 μm will be 4.0×10⁻²/μm² or less in the Ni-based alloy produced. Notethat the specific heat treatment may be performed multiple times.

[Preferable Form (3) of Ni-Based Alloy of Second Embodiment]

In the above-described Ni-based alloy, the grain size number conformingto ASTM E112 may be 0.0 or more, and the total number of Nb carbonitridewhose maximum length is 1 to 100 μm may be 4.0×10⁻²/μm² or less.

In this case, preferably, hot working at an area reduction ratio of35.0% or more is performed at least one time for the Ni-based alloystarting material which has been heated to 1000 to 1300° C. in theabove-described segregation reducing step, and the heat treatment inwhich the holding temperature is 1000 to 1300° C. and the holding timeis 1.0 hour or more is performed at least one time in theabove-described segregation reducing step.

Example 1

A liquid alloy was melted by electric furnace melting. The melted liquidalloy was solidified by a continuous casting process or an ingot-makingprocess to produce a Ni-based alloy starting material (cast piece oringot) having the chemical composition shown in Table 1. The Ni-basedalloy starting materials of Test Nos. 1 to 5 and 8 were cast pieces. Thesection perpendicular to the longitudinal direction of the cast piecewas 600×285 mm. The Ni-based alloy starting materials of Test Nos. 6 and7 were ingots. The section perpendicular to the longitudinal directionof the ingot was 500 mm×500 mm.

TABLE 1 Test Chemical composition (unit is mass %, the balance being Niand impurities) No. C Si Mn P S Cr Mo Nb Ta Nb + Ta Ti Al Fe N O Co Cu 10.021 0.12 0.19 0.012 0.0003 20.9 8.4 3.22 0.002 3.222 0.16 0.15 4.500.006 0.0010 0.57 0.09 2 0.016 0.17 0.16 0.010 0.0002 21.0 8.6 3.230.002 3.232 0.18 0.27 4.10 0.007 0.0009 0.51 0.08 3 0.021 0.12 0.190.012 0.0003 20.9 8.4 3.22 0.002 3.222 0.16 0.15 4.50 0.006 0.0010 0.570.09 4 0.016 0.17 0.16 0.010 0.0002 21.0 8.6 3.23 0.002 3.232 0.18 0.274.10 0.007 0.0009 0.51 0.08 5 0.018 0.06 0.15 0.010 0.0002 21.2 8.9 3.700.002 3.702 0.20 0.17 3.89 0.007 0.0011 0.58 0.09 6 0.019 0.05 0.150.012 0.0003 21.2 8.5 3.28 0.002 3.282 0.18 0.15 3.49 0.011 0.0022 0.480.09 7 0.019 0.05 0.15 0.012 0.0003 21.2 8.5 3.28 0.002 3.282 0.18 0.153.49 0.011 0.0022 0.48 0.09 8 0.019 0.05 0.15 0.012 0.0003 21.2 8.5 3.280.002 3.282 0.18 0.15 3.49 0.011 0.0022 0.48 0.09

A dendrite secondary arm spacing D_(II) was measured by the followingmethod for the produced Ni-based alloy starting material (cast piece) todetermine a solidification cooling rate V_(R) (° C./min) of the Ni-basedalloy starting material of each test number. Specifically, a sample wascollected at a W/4 depth position of a cross section perpendicular tothe longitudinal direction at a longitudinal central position of theNi-based alloy starting material. Of the surface of the sample, asurface parallel with the above-described cross section was subjected tomirror polishing, and was thereafter etched with aqua regia. The etchedsurface was observed by an optical microscope of 400 times magnificationto generate a photographic image of an observation field of view of 200μm×200 μm. Using the obtained photographic image, dendrite secondary armspacings (μm) at arbitrary 20 locations in the observation field of viewwere measured. An average of measured dendrite secondary arm spacingswas defined as a dendrite secondary arm spacing D_(II) (μm). Bysubstituting the obtained dendrite secondary arm spacing D_(II) forFormula (A), a solidification cooling rate V_(R) (° C./min) wasdetermined.

D_(II)=182V_(R) ^(−0.294)  (A)

Further, the segregation reducing step shown in Table 2 was performed onthe Ni-based alloys of Test Nos. 2 to 5, 7, and 8. In Test Nos. 2 and 3,the heat treatment was performed one time as the segregation reducingstep. In Test No. 4, the heat treatment was performed (Heat treatment1), thereafter, hot rolling was performed (Hot working 1), and the heattreatment was performed again (Heat treatment 2) after the hot rolling.In Test No. 5, Heat treatment 1, Hot working 1, Heat treatment 2, Hotworking 2 (hot rolling), and Heat treatment 3 were performed in thisorder. In Test No. 7, Heat treatment 1 was performed. In Test No. 8,Heat treatment 1, Hot working 1, and Heat treatment 2 were performed inthis order. That is, in Test Nos. 2, 3, and 7, only heat treatment atone time was performed. In Test No. 4, heat treatment at one time andcomplex treatment at one time were performed. In Test No. 5, heattreatment at one time and complex treatment at two times were performed.In Test No. 8, complex treatment at one time was performed. Note that inTest Nos. 1 and 6, the segregation reducing step was not performed.

Note that, in all of Test Nos. 4, 5, and 8, a solid material (that is,round-bar) having a cross section of circular shape was produced.Moreover, in all of Test Nos. 4, 5, and 8, Hot working 1 was performedsoon after Heat treatment 1 was performed. In Test No. 5, Hot working 2was performed soon after Heat treatment 2 was performed.

TABLE 2 Segregation reducing step Hot Hot Casting working 1 working 2step Heat treatment 1 Area Heat treatment 2 Area Heat treatment 3 TestV_(R) Temperature Time reduction Temperature Time reduction TemperatureTime No. [° C./min] [° C.] [hr] ratio [%] [° C.] [hr] ratio [%] [° C.][hr] 1 5 — — — — — — — — 2 5 1200 36 — — — — — — 3 5 1200 96 — — — — — —4 5 1200 48 47.3 1200 24 — — — 5 5 1200 48 47.3 1200 24 85 1200 0.08 6 2— — — — — — — — 7 2 1200 150 — — — — — — 8 2 1200 0.83 39.2 1200 85 — —— Mo low- Average Mo Maximum Mo concentration Corrosion Testconcentration concentration region rate No. F1 [%] [%] fraction [%] SSRTtest result [mm/month] 1 −0.62 8.4 11.8 4.0 With sub-crack 0.118 2 −0.218.6 9.3 2.5 With sub-crack 0.124 3 0.06 8.4 9.1 1.9 Without sub-crack0.058 4 0.33 8.6 9.1 0.5 Without sub-crack 0.030 5 0.38 8.9 9.4 0.0Without sub-crack 0.027 6 −0.82 8.5 13.6 8.0 With sub-crack 0.126 7 0.048.5 10.0 1.2 Without sub-crack 0.033 8 0.07 8.5 9.0 0.0 Withoutsub-crack 0.032

The holding temperature (° C.) and the holding time (hr) in each Heattreatment 1 to 3 were as shown in Table 2. The area reduction ratioRd_(n-1)(%) in each Hot working 1, 2 was as shown in Table 2. Moreover,in each test number, F1 (=the right hand side of Formula (1)−the lefthand side of Formula (1)) was determined. Determined F1 is shown inTable 2.

[Evaluation Test] [Mo Concentration Measurement Test]

A sample for Mo concentration measurement test was collected in asection perpendicular to the longitudinal direction (cross section) ofthe Ni-based alloy of each test number after the segregation reducingstep. Specifically, in each test number, a sample was collected from aW/4 depth position of the cross section. Out of the surfaces of thesample, the surface (observation surface) corresponding to the crosssection was mirror polished, and thereafter line analysis by EPMA wasperformed with a beam diameter: 10 μm, a scanning length: 2000 μm, anirradiation time for one point: 3000 ms, and an irradiation pitch: 5 μmin an arbitrary field of view in the observation surface. In thescanning range of 2000 μm in which line analysis was performed, anaverage value of multiple Mo concentrations measured at a 5 μm pitch,and a maximum value of Mo concentration of the measured, multiple Moconcentrations were determined. Further, in the scanning length 2000 μmwhich was the measurement range, a total length (that is, a total lengthof Mo low-concentration region) of ranges in which measured points atwhich the Mo concentration had turned out to be less than 8.0% werecontinuous (ranges in which two or more points were continuous) wasdetermined. The determined total length of Mo low-concentration regionwas used to determine a fraction of Mo low-concentration region (%) bythe following formula.

Fraction of Mo low-concentration region=Total length of Molow-concentration region (μm)/scanning length (2000 μm)×100

[Slow Strain Rate Tensile Test (SSRT)]

In a section perpendicular to the longitudinal direction of the Ni-basedalloy of each Test No. after the segregation reducing step, aslow-strain-rate tensile test specimen was collected from the sameposition as the sample collection position in the Mo concentrationmeasurement test. The length of the slow-strain-rate tensile testspecimen was 80 mm, the length of a parallel part was 25.4 mm, and thediameter of the parallel part was 3.81 mm. The longitudinal direction ofthe slow-strain-rate tensile test specimen was parallel with thelongitudinal direction of the Ni-based alloy. The slow strain ratetensile test (SSRT) was performed at a strain rate of 4.0×10⁻⁶ S⁻¹ whileimmersing the slow-strain-rate tensile test specimen in a 25% NaCl+0.5%CH₃COOH water solution of pH 2.8 to 3.1 and 232° C., which is saturatedwith 0.7 MPa of hydrogen sulfide, to cause the test specimen to be tornoff. In the test specimen after the test, whether or not any sub-crackhad occurred in a portion other than the torn-off part was visuallyconfirmed. When any sub-crack had occurred, it was judged that stresscorrosion cracking had occurred, and when no sub-crack was confirmed, itwas judged that no stress corrosion cracking had occurred, and thereforeexcellent corrosion resistance (SCC resistance) had been achieved.

[Grain Boundary Corrosion Test]

In a section perpendicular to the longitudinal direction of the Ni-basedalloy or each test number after the segregation reducing step, a samplewas collected from the same position as the sample collection positionin the Mo concentration measurement test. The size of test specimen was40 mm×10 mm×3 mm. The collected specimen was used to perform a corrosiontest specified by ASTM G28 Method A. Specifically, the weight of thetest specimen before starting the corrosion test was measured. After themeasurement, the test specimen was immersed in a 50% sulfuricacid/ferric sulfate solution for 120 hours. After elapse of 120 hours,the weight of the test specimen after the test was measured. From thechange in weight of the measured test specimen, a corrosion rate(mm/month) of each test specimen was determined.

[Test Results]

Test results are shown in Table 2. Referring to Table 2, in Test Nos. 3to 5, 7, and 8, the chemical composition of the Ni-based alloy wasappropriate, and F1 was 0 or more, thus satisfying Formula (1) in thesegregation reducing step. For that reason, in a section perpendicularto the longitudinal direction of the Ni-based alloy, the averageconcentration of Mo was 8.0% or more in mass %, the maximum value of Moconcentration was 11.0% or less in mass %, and further the area fractionof regions in which Mo concentration was less than 8.0% in mass % (thefraction of Mo low-concentration region) was less than 2.0%. As aresult, no sub-crack was confirmed in the SSRT test. Further, thecorrosion rate was 0.075 mm/month or less, thus exhibiting excellentcorrosion resistance. Note that in the Ni-based alloys of Test Nos. 3 to5, 7, and 8, the total number of Nb carbonitride whose maximum lengthwas 1 to 100 μm was 4.0×10⁻²/μm² or less.

Further, in Test Nos. 4, 5, and 8, hot working was performed before thefinal heat treatment in the segregation reducing step. As a result ofthat, compared with Test No. 3 in which hot working was not performedbefore heat treatment, the corrosion rate further decreased to be 0.055mm/month or less.

On the other hand, in Test Nos. 1 and 6, the segregation reducing stepwas not performed after the Ni-based alloy starting material wasproduced by the casting step. For that reason, in a sectionperpendicular to the longitudinal direction of the Ni-based alloy, themaximum value of Mo concentration was more than 11.0% in mass %, andfurther the area fraction of regions in which Mo concentration was lessthan 8.0% in mass % (the fraction of Mo low-concentration region) was2.0% or more. As a result of that, the sub-crack was confirmed in theSSRT test. Further, the corrosion rate was more than 0.075 mm/month.

In Test No. 2, although the heat treatment was performed in thesegregation reducing step, F1 was less than 0, and did not satisfyFormula (1). For that reason, the fraction of Mo low-concentrationregion was 2.0% or more. As a result, the sub-crack was confirmed in theSSRT test. Further, the corrosion rate was more than 0.075 mm/month.

Example 2

The liquid alloy which was melted by electric furnace melting wassolidified by a continuous casting process or ingot-making process toproduce Ni-based alloy starting materials (cast pieces or ingots) havingthe chemical compositions of Table 3. The Ni-based alloy startingmaterials of Test Nos. 9 to 21 were cast pieces, and the section (crosssection) perpendicular to the longitudinal direction of each cast piecewas 600×285 mm. Note that in the F2 column of Table 3, F2 values(=(Ca+Nd+B)/S) of each test number are listed. Note that blank portionsin Table 3 indicate that the content of a corresponding element wasbelow a detection limit.

TABLE 3 Chemical composition (unit is mass %, the balance being Ni andimpurities) Test Nb + No. C Si Mn P S Cr Mo Nb Ta Ta Ti Al 9 0.014 0.110.21 0.012 0.0003 21.5 8.5 3.30 3.300 0.22 0.11 10 0.016 0.07 0.19 0.0070.0004 21.4 8.5 3.42 3.420 0.19 0.08 11 0.016 0.17 0.16 0.010 0.000221.0 8.6 3.23 0.002 3.232 0.18 0.27 12 0.018 0.06 0.15 0.010 0.0002 21.28.9 3.70 0.002 3.702 0.20 0.17 13 0.020 0.11 0.21 0.011 0.0005 21.5 8.63.36 3.360 0.20 0.09 14 0.020 0.14 0.20 0.0005 21.5 8.6 3.36 3.360 0.190.10 15 0.020 0.12 0.21 0.004 0.0006 21.5 8.5 3.32 3.321 0.20 0.11 160.019 0.11 0.21 0.011 0.0004 21.5 8.6 3.39 3.390 0.21 0.10 17 0.018 0.130.21 0.004 0.0004 21.5 8.6 3.40 3.400 0.20 0.10 18 0.020 0.15 0.20 0.0040.0005 21.4 8.6 3.38 3.380 0.19 0.10 19 0.021 0.12 0.21 0.005 0.000521.6 8.6 3.37 3.370 0.21 0.11 20 0.020 0.16 0.20 0.005 0.0005 21.5 8.53.34 3.340 0.19 0.10 21 0.017 0.10 0.21 0.009 0.0003 21.6 8.6 3.44 3.4400.18 0.11 Chemical composition (unit is mass %, the balance being Ni andimpurities) Test Ca + No. Fe N O Co Cu Ca Nd B Nd + B F2 9 3.02 0.0110.0021 0.01 0.01 0.0000 0.0 10 2.99 0.013 0.0013 0.04 0.01 0.0000 0.0 114.10 0.007 0.0009 0.51 0.08 0.0005 0.0005 2.0 12 3.89 0.007 0.58 0.090.0007 0.0007 2.8 13 2.94 0.012 0.0100 0.0001 0.0001 0.6 14 3.03 0.0120.0040 0.0001 0.0001 0.6 15 3.03 0.011 0.0050 0.0001 0.0001 0.5 16 3.020.012 0.0090 0.014 0.0001 0.0141 8.5 17 3.01 0.011 0.0050 0.035 0.00010.0351 20.2 18 3.02 0.011 0.0110 0.031 0.0019 0.0329 25.1 19 3.05 0.0240.0070 0.390 0.0021 0.3921 185.9 20 3.02 0.012 0.0110 0.350 0.00170.3517 165.7 21 3.67 0.014 0.0010 0.01 0.0001 0.0001 1.0

For the produced Ni-based alloy starting materials (cast pieces), thedendrite secondary arm spacing D_(II) was measured by theabove-described method to determine the solidification cooling rateV_(R) (° C./min) of the Ni-based alloy starting material of each testnumber. As a result, as shown in Table 4, the solidification coolingrate V_(R) was 5 (° C./min) in all the test numbers.

TABLE 4 Segregation reducing step Hot Hot Casting working 1 working 2step Heat treatment 1 Area Heat treatment 2 Area Heat treatment 3 TestV_(R) Temperature Time reduction Temperature Time reduction TemperatureTime No. [° C./min] [° C.] [hr] ratio [%] [° C.] [hr] ratio [%] [° C.][hr] F1 9 5 1200 96 — — — — — — 0.06 10 5 1200 48 47.3 1200 24 — — —0.33 11 5 1200 96 — — — — — — 0.06 12 5 1200 48 47.3 1200 24 — — — 0.3313 5 1200 48 47.3 1200 24 — — — 0.33 14 5 1200 48 47.3 1200 24 — — —0.33 15 5 1200 48 47.3 1200 24 — — — 0.33 16 5 1200 48 47.3 1200 24 — —— 0.33 17 5 1200 48 47.3 1200 24 — — — 0.33 18 5 1200 48 47.3 1200 24 —— — 0.33 19 5 1200 48 47.3 1200 24 85.0 1200 0.08 0.38 20 5 1200 48 47.31200 24 85.0 1200 0.08 0.38 21 5 1200 48 47.3 1200 24 85.0 1200 0.080.38 Reduction Average Mo Maximum Mo Mo low- area after Testconcentration concentration concentration Corrosion rate fraction No. F2[%] [%] fraction [%] SSRT test result [mm/month] [%] 9 0.0 8.3 9.4 1.4Without corrosion 0.030 24.9 10 0.0 8.6 9.5 0.9 Without corrosion 0.02824.7 11 2.0 8.3 9.4 1.4 Without corrosion 0.030 50.1 12 2.8 8.6 9.5 0.9Without corrosion 0.028 70.6 13 0.6 8.6 9.5 0.9 Without corrosion 0.02831.3 14 0.6 8.6 9.5 0.9 Without corrosion 0.028 30.0 15 0.5 8.6 9.5 0.9Without corrosion 0.028 31.7 16 8.5 8.6 9.5 0.9 Without corrosion 0.02883.2 17 20.2 8.6 9.5 0.9 Without corrosion 0.028 80.1 18 25.1 8.6 9.50.9 Without corrosion 0.028 85.9 19 185.9 8.5 9.1 0.5 Without corrosion0.029 82.4 20 165.7 8.5 9.1 0.5 Without corrosion 0.029 84.4 21 1.0 8.59.1 0.5 Without corrosion 0.029 34.2

The segregation reducing step was performed on the Ni-based alloy ofeach test number. Specifically, in Test Nos. 9 and 11, the heattreatment was performed only one time, and the hot working step was notperformed. The holding temperature of the heat treatment was 1200° C.,and the holding time was 96 hours. As a result, each F1 was 0.06, thussatisfying Formula (1).

In any of Test Nos. 10 and 12 to 18, the heat treatment was performed(Heat treatment 1), thereafter hot rolling was performed (Hot working1), and the heat treatment was performed again after the hot rolling(Heat treatment 2). The holding temperature in Heat treatment 1 was1200° C., and the holding time was 48 hours. The area reduction ratio inHot working 1 was 47.3%. The holding temperature in Heat treatment 2 was1200° C. and the holding time was 24 hours. As a result, each F1 (=theright hand side of Formula (1)−the left hand side of Formula (1)) was0.33, thus satisfying Formula (1).

In Test Nos. 19 to 21, Heat treatment 1, Hot working 1, Heat treatment2, Hot working 2, and Heat treatment 3 were performed in this order. Theholding temperature of Heat treatment 1 was 1200° C., and the holdingtime was 48 hours. The cumulative area reduction ratio in Hot working 1was 47.3%. The holding temperature in Heat treatment 2 was 1200° C., andthe holding time was 24 hours. The cumulative area reduction ratio inHot working 2 was 85.0%. The holding temperature in Heat treatment 3 was1200° C., and the holding time was 0.08 hours. As a result, each F1 was0.38, thus satisfying Formula (1).

By the steps described above, Ni-based alloys of Test Nos. 9 to 21 wereproduced. Note that in all of Test Nos. 9 to 21, secondary melting wasnot performed on the Ni-based all starting material after the castingstep. The Ni-based alloys of Test Nos. 9 and 11 were cast pieces, andthe Ni-based alloys of Test Nos. 10, and 12 to 21 were each a solidmaterial (that is a round-bar) which had a cross section of a circularshape. Note that in Test Nos. 10, and 12 to 21, Hot working 1 wasperformed soon after Heat treatment 1 was performed. In Test Nos. 19 to21, Hot working 2 was performed soon after Heat treatment 2 wasperformed.

[Hot Workability Evaluation Test]

The Ni-based alloy of each test number was used to perform the followingtensile test. Tensile test specimens were collected from the Ni-basedalloys. The tensile test specimen corresponded to 14A test specimen ofJIS standard. In each test number, a tensile test specimen was collectedfrom a W/4 depth position of a cross section. The tensile test specimenwas heated to 900° C. By using a tensile test specimen of 900° C.,tensile test was performed at a strain rate of 10/sec in the atmosphereto measure reduction area after fraction (%). When the reduction areaafter fraction was 35.0% or more, it was judged that hot workability wasexcellent. Measurement results are shown in Table 3.

[Test Results]

Referring to Table 3, all of Test Nos. 9 to 21 satisfied Formula (1).For that reason, in a section perpendicular to the longitudinaldirection of the Ni-based alloy, the average concentration of Mo was8.0% or more in mass %, the maximum value of Mo concentration was 11.0%or less in mass %, and further the area fraction of regions in which Moconcentration was less than 8.0% in mass % was less than 2.0%. As aresult, no sub-crack was confirmed in the SSRT test. Further, thecorrosion rate was 0.075 mm/month or less, thus exhibiting excellentcorrosion resistance. Note that in the Ni-based alloys of Test Nos. 9 to21, a total number of Nb carbonitride whose maximum length was 1 to 100μm was 4.0×10⁻²/μm² or less.

Further, in all of Test Nos. 11, 12, and 16 to 20, the chemicalcompositions were appropriate, and F2 was 2.0 or more, thus satisfyingFormula (2). For that reason, all of the reduction area after fractionswere 35.0% or more (more specifically, 45.0% or more), thus exhibitingexcellent hot workability.

Example 3

The grain size numbers of Ni-based alloys of Test No. 5 of Example 1 andTest No. 12 of Example 2 were determined by the following method. TheNi-based alloy was divided into 5 equal sections in the axial directionto identify an axially central position of each section. In eachsection, sample collection positions were identified at a 90 degreepitch around the axis (around the longitudinal direction) at an axiallycentral position. Samples were collected from the W/4 depth positions ateach identified sample collection position. The observation surface ofsample was a section perpendicular to the axial direction of theNi-based alloy, and the area of the observation surface was 40 mm².According to the above-described method, 4 samples per each section, and20 samples in all the sections were collected. The observation surfaceof each collected sample was etched by using the Kalling's reagent tocause grain boundaries in the surface to appear. Observing the etchedobservation surface, the grain size number was determined conforming toASTM E112. An average value of the grain size numbers determined from 20samples was defined as the grain size number conforming to ASTM E112 inan Ni-based alloy.

As a Comparative Example, a Ni-based alloy starting material of Test No.22 having the chemical composition shown in Table 5 was prepared. TheNi-based alloy starting material was a cast piece, a sectionperpendicular to the longitudinal direction of the cast piece was600×285 mm. The chemical composition of Test No. 22 was the same as thatof Test No. 5.

TABLE 5 Test Chemical composition (unit is mass %, the balance being Niand impurities) No. C Si Mn P S Cr Mo Nb Ta Nb + Ta Ti Al 22 0.018 0.060.15 0.010 0.0002 21.2 8.9 3.70 0.002 3.702 0.20 0.17 5 0.018 0.06 0.150.010 0.0002 21.2 8.9 3.70 0.002 3.702 0.20 0.17 12 0.018 0.06 0.150.010 0.0002 21.2 8.9 3.70 0.002 3.702 0.20 0.17 Chemical composition(unit is mass %, the balance being Ni and impurities) Test Ca + No. Fe NO Co Cu Ca Nd B Nd + B F2 22 3.89 0.007 0.001 0.58 0.09 5 3.89 0.0070.001 0.58 0.09 12 3.89 0.007 0.58 0.09 0.0007 0.0007 2.8

For the Ni-based alloy starting material (cast piece) of Test No. 22,the dendrite secondary arm spacing D_(II) was measured by the samemethod as in Example 1 to determine the solidification cooling rateV_(R) (° C./min) of the Ni-based alloy starting material of each testnumber. As a result, the solidification cooling rate V_(R) was 5° C./minas shown in Table 6.

TABLE 6 Segregation reducing step Hot Hot working 1 working 2 CastingCumulative Cumulative step Heat treatment 1 area Heat treatment 2 areaHeat treatment 3 Grain Test V_(R) Temperature Time reduction TemperatureTime reduction Temperature Time size No. [° C./min] [° C.] [hr] ratio[%] [° C.] [hr] ratio [%] [° C.] [hr] F1 number 22 5 1200 48 31.3 120024 62.6 1200 0.08 0.30 −2.0 5 5 1200 48 47.3 1200 24 85.0 1200 0.08 0.382.0 12 5 1200 48 47.3 1200 24 — — — 0.33 0.0

For the Ni-based alloy starting material of Test No. 22, the segregationreducing step as shown in Table 6 was performed. Compared with theproduction conditions of Test No. 5, the area reduction ratio of thefirst hot working was 31.3%. Moreover, the cumulative area reductionratio of the second hot working was 62.6%, and the area reduction ratioin the second hot working was 31.3%. That is, in Test No. 22, both thearea reduction ratios in each hot working were less than 35.0%. For TestNo. 22 as well, the grain size number was determined by the same methodas in Test No. 5.

As a result of determining the grain size number, in Test No. 5, thegrain size number conforming to ASTM E112 was 0.0 or more (2.0), and inTest No. 12, the grain size number conforming to ASTM E112 was 0.0. Onthe other hand, in Test No. 22, the grain size number conforming to ASTME112 was less than 0.0 (−2.0).

Example 4

The total number of coarse Nb carbonitride of the Ni-based alloy of TestNo. 4 of Example 1 was determined by the following method. The Ni-basedalloy was divided into 5 equal sections in the axial direction and anaxially central position of each section was identified. In eachsection, sample collection positions were identified at a 90 degreepitch around the axis (around the longitudinal direction) at an axiallycentral position. A samples was collected from a wall thickness centralposition at each identified sample collection position. The observationsurface of sample was a section perpendicular to the axial direction ofthe Ni-based alloy. Nb carbonitride was identified by EPMA in anarbitrary one field of view (400 μm×400 μm) in each observation surface(a total of 20). A maximum length of the identified Nb carbonitride wasmeasured. As described so far, among straight lines connecting arbitrarytwo points on the interface between Nb carbonitride and the motherphase, the value of the longest straight line is defined as the maximumlength of the Nb carbonitride. After measuring the maximum length ofeach Nb carbonitride, Nb carbonitride whose maximum length was 1 to 100μm (coarse Nb carbonitride) was identified, and a total number of coarseNb carbonitride in all the 20 fields of view was determined. Based onthe obtained total number, a total number (/μm²) of coarse Nbcarbonitride was determined.

As a Comparative Example, a Ni-based alloy of Test No. 23 shown in Table7 was prepared. The Ni-based alloy starting material was a cast piece, asection perpendicular to the longitudinal direction of the cast piecewas 600×285 mm. The chemical composition of Test No. 23 was the same asthat of Test No. 4.

TABLE 7 Chemical composition (unit is mass %, the balance being Ni andimpurities) Test Nb + No. C Si Mn P S Cr Mo Nb Ta Ta Ti Al Fe N O Co Cu23 0.016 0.17 0.16 0.010 0.0002 21.0 8.6 3.23 0.002 3.232 0.18 0.27 4.100.007 0.0009 0.51 0.08 4 0.016 0.17 0.16 0.010 0.0002 21.0 8.6 3.230.002 3.232 0.18 0.27 4.10 0.007 0.0009 0.51 0.08

For the Ni-based alloy starting material of Test No. 23, the segregationreducing step shown in Table 8 was performed. Specifically, in Test No.23, the first heat treatment (Heat treatment 1) was performed at thesame temperature as in Test No. 4, and thereafter, hot rolling (Hotworking 1) was performed at an area reduction ratio as in Test No. 4,and second heat treatment (Heat treatment 2) was performed again at thesame temperature as in Test No. 4, after the hot rolling. However, theholding times in Heat treatment 1 and Heat treatment 2 were both 50minutes (0.83 hours), and were less than 1 hour. In Test No. 23 as well,as in Test No. 4, the total number of coarse Nb carbonitride wasdetermined.

TABLE 8 Segregation reducing step Hot Total Casting working 1 number ofReduction step Heat treatment 1 Cumulative Heat treatment 2 coarse Nbarea after Test V_(R) Temperature Time area reduction Temperature Timecarbonitride fraction No. [° C./min] [° C.] [hr] ratio [%] [° C.] [hr]F1 (/μm²) [%] 23 5 1200 0.83 47.3 1200 0.83 −0.47 0.13 13.2 4 5 1200 4847.3 1200 24 0.33 5.2 × 10⁻³ 69.6

Further, for the Ni-based alloys of Test Nos. 4 and 23, the hotworkability evaluation test was performed by the same method as inExample 2 to determine the reduction area after fraction (%).

Although the total number of coarse Nb carbonitride was 4.0×10⁻²/μm² orless in Test No. 4, it was more than 4.0×10⁻²/μm² in Test No. 23. As aresult of that, while the reduction area after fraction became more than35.0% in Test No. 4, the reduction area after fraction was less than35.0% in Comparative Example.

So far, embodiments of the present invention have been described.However, the above-described embodiments are merely examples forpracticing the present invention. Therefore, the present invention willnot be limited to the above-described embodiments and can be practicedby appropriately altering the above-described embodiments within a rangenot departing from the spirit thereof.

1-9. (canceled)
 10. A method for producing a Ni-based alloy, comprising:a casting step of casting a liquid alloy to produce a Ni-based alloystarting material, which has a chemical composition consisting of: inmass %, C: 0.100% or less, Si: 0.50% or less, Mn: 0.50% or less, P:0.015% or less, S: 0.0150% or less, Cr: 20.0 to 23.0%, Mo: 8.0 to 10.0%,one or more elements selected from the group consisting of Nb and Ta:3.150 to 4.150%, Ti: 0.05 to 0.40%, Al: 0.05 to 0.40%, Fe: 0.05 to5.00%, N: 0.100% or less, O: 0.1000% or less, Co: 0 to 1.00%, Cu: 0 to0.50%, one or more elements selected from the group consisting of Ca,Nd, and B: 0 to 0.5000%, and the balance being Ni and impurities; and asegregation reducing step of performing, on the Ni-based alloy startingmaterial produced by the casting step, heat treatment, or the heattreatment and, after the heat treatment, complex treatment including hotworking and heat treatment after the hot working, to satisfy Formula(1):      [Expression  1] $\begin{matrix}{V_{R}^{- 0.294} \leq {1.27 \times 10^{3}\mspace{14mu} {\sum\limits_{n = 1}^{N}\; \sqrt{\left( {1 - \frac{{Rd}_{n - 1}}{100}} \right)^{- 1} \cdot {\exp \left( \frac{{- 2.89} \times 10^{4}}{T_{n} + 273} \right)} \cdot t_{n}}}}} & (1)\end{matrix}$ where, each symbol in Formula (1) is as follows: V_(R):Solidification cooling rate (° C./min) of the liquid alloy in thecasting step, T_(n): Holding temperature (° C.) in the n-th heattreatment, t_(n): Holding time (hr) at the holding temperature in then-th heat treatment, Rd_(n-1): Cumulative area reduction ratio (%) ofthe Ni-based alloy starting material before the n-th heat treatment, andN: Total number of the heat treatment.
 11. The method for producing aNi-based alloy according to claim 10, wherein the holding temperature is1000 to 1300° C.
 12. The method for producing a Ni-based alloy accordingto claim 11, wherein in the segregation reducing step, the complextreatment is performed one or more times, and hot working is performedat least one time at an area reduction ratio of 35.0% or more on theNi-based alloy starting material which has been heated to 1000 to 1300°C.
 13. The method for producing a Ni-based alloy according to claim 11,wherein in the segregation reducing step, the heat treatment in whichthe holding temperature is 1000 to 1300° C. and the holding time is 1.0hour or more is performed at least one time.
 14. The method forproducing a Ni-based alloy according to claim 12, wherein in thesegregation reducing step, the heat treatment in which the holdingtemperature is 1000 to 1300° C. and the holding time is 1.0 hour or moreis performed at least one time.
 15. The method for producing a Ni-basedalloy according to claim 10, wherein the chemical composition containsone or more elements selected from the group consisting of Ca, Nd, and Bby a content that satisfies Formula (2):(Ca+Nd+B)/S≥2.0  (2) where, each symbol of element in Formula (2) issubstituted by a content in atomic % (at %) of a corresponding element.16. The method for producing a Ni-based alloy according to claim 11,wherein the chemical composition contains one or more elements selectedfrom the group consisting of Ca, Nd, and B by a content that satisfiesFormula (2):(Ca+Nd+B)/S≥2.0  (2) where, each symbol of element in Formula (2) issubstituted by a content in atomic % (at %) of a corresponding element.17. The method for producing a Ni-based alloy according to claim 12,wherein the chemical composition contains one or more elements selectedfrom the group consisting of Ca, Nd, and B by a content that satisfiesFormula (2):(Ca+Nd+B)/S≥2.0  (2) where, each symbol of element in Formula (2) issubstituted by a content in atomic % (at %) of a corresponding element.18. The method for producing a Ni-based alloy according to claim 13,wherein the chemical composition contains one or more elements selectedfrom the group consisting of Ca, Nd, and B by a content that satisfiesFormula (2):(Ca+Nd+B)/S≥2.0  (2) where, each symbol of element in Formula (2) issubstituted by a content in atomic % (at %) of a corresponding element.19. The method for producing a Ni-based alloy according to claim 14,wherein the chemical composition contains one or more elements selectedfrom the group consisting of Ca, Nd, and B by a content that satisfiesFormula (2):(Ca+Nd+B)/S≥2.0  (2) where, each symbol of element in Formula (2) issubstituted by a content in atomic % (at %) of a corresponding element.20. A Ni-based alloy, comprising a chemical composition consisting of:in mass %, C: 0.100% or less, Si: 0.50% or less, Mn: 0.50% or less, P:0.015% or less, S: 0.0150% or less, Cr: 20.0 to 23.0%, Mo: 8.0 to 10.0%,one or more elements selected from the group consisting of Nb and Ta:3.150 to 4.150%, Ti: 0.05 to 0.40%, Al: 0.05 to 0.40%, Fe: 0.05 to5.00%, N: 0.100% or less, O: 0.1000% or less, Co: 0 to 1.00%, Cu: 0 to0.50%, one or more elements selected from the group consisting of Ca,Nd, and B: 0 to 0.5000%, and the balance being Ni and impurities,wherein in a section perpendicular to a longitudinal direction of theNi-based alloy, an average concentration of Mo is 8.0% or more in mass%; a maximum value of the Mo concentration is 11.0% or less in mass %;and further an area fraction of a region in which the Mo concentrationis less than 8.0% in mass % is less than 2.0%.
 21. The Ni-based alloyaccording to claim 20, wherein the chemical composition contains one ormore elements selected from the group consisting of Ca, Nd, and B by acontent that satisfies Formula (2):(Ca+Nd+B)/S≥2.0  (2) where, each symbol of element in Formula (2) issubstituted by a content in atomic % (at %) of a corresponding element.22. The Ni-based alloy according to claim 20, wherein a grain sizenumber conforming to ASTM E112 is 0.0 or more.
 23. The Ni-based alloyaccording to claim 21, wherein a grain size number conforming to ASTME112 is 0.0 or more.
 24. The Ni-based alloy according to claim 20,wherein a total number of Nb carbonitride whose maximum length is 1 to100 μm is 4.0×10⁻²/μm² or less in the Ni-based alloy.
 25. The Ni-basedalloy according to claim 21, wherein a total number of Nb carbonitridewhose maximum length is 1 to 100 μm is 4.0×10⁻²/μm² or less in theNi-based alloy.
 26. The Ni-based alloy according to claim 22, wherein atotal number of Nb carbonitride whose maximum length is 1 to 100 μm is4.0×10⁻²/μm² or less in the Ni-based alloy.
 27. The Ni-based alloyaccording to claim 23, wherein a total number of Nb carbonitride whosemaximum length is 1 to 100 μm is 4.0×10⁻²/μm² or less in the Ni-basedalloy.